CHATTANOOGA, Tenn. — Federal investigators on Friday dug into the background and travels of the 24-year-old gunman who they say killed four Marines in an attack on two military sites here, focusing on a seven-month trip he made last year to Jordan and scouring his electronic trail in search of a motive for the killings.

The crucial, unanswered question was whether Mohammod Youssuf Abdulazeez, a naturalized U.S. citizen who was born in Kuwait, came into contact with, or was inspired by, any Islamist extremist groups, intelligence officials said.

Federal agents flew Abdulazeez’s computer, cellphone and other electronics to Washington for forensic analysis of his communications. They also worked with local law enforcement following up on 70 leads about his activities, and they asked intelligence services in Jordan and Kuwait about his movements there.

Abdulazeez, who was killed in a gunbattle with the police during Thursday’s attack, was the son of Palestinians from Jordan, and made several trips to Jordan and Kuwait, where he had relatives, officials said, but he spent most of his life in southeastern Tennessee, and graduated from college here with a degree in engineering.

“We are exploring all travel he has done, and we have asked our intelligence partners throughout the world to provide us with any information they may have as to travel and activities,” said Edward W. Reinhold, the agent in charge of the Knoxville office of the FBI, which is leading the investigation. “It would be premature to speculate on exactly why the shooter did what he did. However, we are conducting a thorough investigation to determine whether this person acted alone, was inspired or directed.”

Officials said there was no indication so far of any links to terror groups, leaving them to wonder how a young man with no known history of violence or radicalism turned up Thursday with several weapons, spraying bullets at Americans in uniform. Some “lone wolf” attacks have been carried out by people who had no direct contact with extremist groups, but they were influenced by messages online, like those from the Islamic State urging Muslims to take up arms and attack U.S. military sites.

“This attack raises several questions about whether he was directed by someone or whether there’s enough propaganda out there to motivate him to do this,” said a senior U.S. intelligence official, who spoke on the condition of anonymity because the investigation was still underway.

A congressional official who was briefed on the investigation said it was not yet clear whether Abdulazeez’s computer or communications were encrypted, which would lengthen the time needed to pry clues out of them. Just days before the attacks Thursday, Abdulazeez began a blog where he posted about Islam, according to the SITE Intelligence Group, which tracks international terrorist groups. He at one point compared life to a prison and in another point called life “short and bitter.”

New details emerged of the bloody day that shook this city, with officials describing a furious firefight between Abdulazeez — armed, some federal officials said, with an AK-47 assault rifle — and Chattanooga police officers as they battled at the naval reserve center where the four Marines were killed.

Reinhold of the FBI said the gunman “did have at least two long guns,” meaning rifles or shotguns, “and he did have one handgun that we’re aware of.” He said some of the guns were legal and some may not have been. Abdulazeez, he said, did not have body armor, but wore a vest with multiple ammunition magazines.

The gunman fired first on an armed services recruiting center in a strip mall here, and then, pursued by Chattanooga police officers, raced in a Ford Mustang convertible to the naval reserve facility, a fenced-in campus with buildings and a tree-lined parking lot, and opened fire. “All indications are that he was killed by fire from the Chattanooga police officers,” Reinhold said.

Fred Fletcher, the Chattanooga police chief, said that as soon as the call went out on the radio, officers swarmed the scene, including members of his command staff who bolted from a meeting, and others who were off-duty at home and responded in civilian clothes.

“It was clear that this gunman had every intent to encounter and murder police officers,” he said. When one officer at the second shooting scene was shot in the ankle — one of three people injured in the battle — the other officers there “put their hands on him, dragged him from the gunfire, and bravely returned fire,” Fletcher said.

William C. Killian, the U.S. attorney for the Eastern District of Tennessee, said, “We will continue to investigate it as an act of terrorism until the proof shows us otherwise.”

A day after the shootings, the city mourned. About 600 people attended a memorial service at Olivet Baptist Church for the slain marines and wounded naval officer. The speakers, including Gov. Bill Haslam, said the attack would not drive a wedge between members of this diverse city, which has a sizable Muslim population. “The sense of violation that we all feel today cannot be healed individually,” Mayor Andy Berke said. “The pain can only be healed as a community.”

Sailors and Marines in uniform were at the service, some giving the Marine Corps battle call of “hooyah!” each time the bravery of the Marines was invoked.

“I ask you to use this in some way to heal what is broken in Chattanooga, in the state of Tennessee, in this country,” said Sgt. David Jones, a Marine stationed here.

Before his stay in Jordan last year, Abdulazeez, who was a naturalized U.S. citizen and made the trip on a U.S. passport, had traveled at least four other times to the country, for two weeks to two months at a time, said federal law enforcement officials, who were not authorized to speak about the investigation. They said he was in Jordan in the last weeks of 2005, in summer 2008, summer 2010, and spring 2013, when he also spent some time in Canada, returning to the United States in May.

Officials often look at international travel in terrorism cases because training in terrorist camps in Pakistan or Afghanistan has been seen as a crucial step in developing a plot. But a federal official said there was no indication that Abdulazeez’s trips were connected with the shootings, which would have required no special training.

Authorities in Jordan said Abdulazeez traveled there last year to visit a maternal uncle, and the tiny, arid country -- though squeezed into a volatile region, bordering Syria, Israel, Iraq and Saudi Arabia -- is not considered a training ground for terrorism groups.

Jordan’s monarchy heads the Arab world’s most unflinchingly pro-American government and one of two that have standing peace treaties with Israel. Its security services are pervasive and their clampdowns on dissent played a role in keeping the country stable through the Arab Spring uprisings and the subsequent unrest that has rocked many Arab countries.

While Jordan has little internal militant activity, it is home to several prominent Qaida-linked ideologues, and parts of the country are bastions of ultraconservative Islam. Abu Musbab Al-Zarqawi, who led al-Qaida in Iraq before being killed in a U.S. airstrike in 2006, hailed from the town of Zarqa, from which hundreds of young men have left more recently to join jihadists factions in Syria and Iraq.

The official Kuwait News Agency also reported Friday that Abdulazeez visited there 2010 for a few weeks before heading to Jordan.

Born in Kuwait in 1990, Abdulazeez became a U.S. citizen in 2003 through the naturalization of his mother, federal officials said; his father was also naturalized. Because he was a minor, he did not have to apply separately for citizenship. A divorce complaint filed by his mother in 2009 and then withdrawn, said the parents were from “the state of Palestine.”

Counterterrorism officials had not been investigating Abdulazeez before Thursday’s shooting. His father had been investigated about seven years ago, officials said, for possibly giving money to a group affiliated with Hamas, the Islamic militant group in Gaza that the United States and other Western nations consider a terrorist organization. The investigation was closed and no charges were filed. But the father was placed on a watch-list for a time. A similar investigation was conducted in the 1990s and it, too, was closed without charges.

Rep. Michael McCaul, R-Texas, chairman of the House Homeland Security Committee, said the watch list for a time had prevented the elder Abdulazeez from flying. “I believe there was a preliminary investigation, but there was no derogatory information, and he was taken off the list,” he said.

In fact, father and son were able to travel together to Jordan in recent years, a law enforcement official said. It was not clear whether the younger Abdulazeez went to Jordan alone last year.

Abdulazeez’s family had faced difficulties in recent years. His father filed for bankruptcy protection in 2002, and his mother, Rasmia I. Abdulazeez, filed for divorce seven years later, court records show. In the divorce complaint, which was withdrawn within a month, a lawyer for Rasmia Abdulazeez said her husband, Youssuf S. Abdulazeez, had “repeatedly beaten” her and had “on occasion” abused the children by “striking and berating them without provocation or justification.”

The complaint also accused Youssuf Abdulazeez of sexual and verbal abuse, and of declaring his intentions “to take a second wife, as permitted under certain circumstances under Islamic law.”

The lawyer who filed the complaint, John R. Meldorf III, did not respond to a request for comment. Court records did not list a lawyer for Youssuf Abdulazeez.

Mohammod Abdulazeez graduated in 2012 from the University of Tennessee at Chattanooga with a bachelor’s degree in electrical engineering. In college, he worked as an intern for the Tennessee Valley Authority.

In 2013, he worked briefly at a nuclear power plant near Cleveland, where he was dismissed after just 10 days.

FirstEnergy Corp. said Abdulazeez had been “conditionally employed” at its Perry Nuclear Power Plant, near Lake Erie, in May 2013 before officials “determined that he did not meet minimum requirements for ongoing employment.” The company said Abdulazeez had “never entered the secured area of the plant” and that he had had access only to an administrative building.

A company spokesman, Todd M. Schneider, on Friday evening refused to elaborate about what specifically had prompted FirstEnergy to dismiss Abdulazeez.

The only run-in Abdulazeez had with the law in the Chattanooga area appears to have been an April 20 arrest on a charge of driving while intoxicated; he posted a $2,000 bond.

According to a police affidavit, officers spotted him weaving through downtown Chattanooga after 2 a.m., in a gray 2001 Toyota Camry, and when they pulled over, they smelled alcohol and marijuana, and he failed a sobriety test. They said his eyes were bloodshot, his speech was slurred, he was “unsteady on his feet,” and he had “irritated nostrils” and white powder under his nose, which Abdulazeez claimed came from snorting crushed caffeine pills. He was due to appear in court on July 30.