State and federal authorities on Thursday pledged "robust and meticulous" dual investigations into the death of George Floyd, calling the case a top priority that will be monitored directly by President Donald Trump and U.S. Attorney General William Barr.

But the investigations could take some time — maybe more than those protesting for justice in the streets had hoped.

"We are going to investigate it as expeditiously, as thoroughly and completely as justice demands," said Hennepin County Attorney Mike Freeman. "Sometimes that takes a little time. And we ask people to be patient."

Freeman called Floyd's death "senseless," but said moving too quickly could threaten the outcome of the case. He invoked the prosecution of Baltimore officers charged in the death of Freddie Gray, saying the district attorney there moved too quickly and lost the case as a result. "We have to do this right, and that's what we'll do," Freeman said.

Standing in front of the FBI field office in Brooklyn Center, Freeman joined U.S. Attorney Erica MacDonald, FBI special-agent-in-charge Rainer Drolshagen and Bureau of Criminal Apprehension Superintendent Drew Evans. They collectively pledged a "thorough and expeditious" probe into the death of Floyd, three days after officer Derek Chauvin was filmed pressing his knee on Floyd's neck until Floyd lost consciousness during a police call over a suspected counterfeit bill and later died. Chauvin and three other officers at the scene have since been fired, while Minneapolis Mayor Jacob Frey has demanded an arrest and charge in the case.

MacDonald said the federal investigation will focus on whether the officers used their authority as law enforcement agents to deprive Floyd of his constitutional rights. To meet the bar of this federal civil rights violation, investigators must prove the officers took action — or failed to do so — with knowledge of wrongdoing.

"It is imperative that the investigation is done right and done right the first time, and that is what we are going to do," said MacDonald, also signaling the charging decision may not come as swiftly as some had hoped. MacDonald said she learned about Floyd's death just hours after it occurred and has been working nonstop with the FBI since. "We understand the severity of the situation unfolding. It breaks my heart to see what is going on in the streets of Minneapolis, St. Paul and the suburbs, and I am pleading with individuals to remain calm and let us conduct this investigation."

MacDonald and Drolshagen said their respective offices along with the Justice Department's Civil Rights Division "are conducting a robust criminal investigation into the circumstances surrounding the May 25, 2020, death of George Floyd."

Freeman, whose office will handle any state criminal charges against the officers involved, said his office has been flooded with as many as 1,000 calls daily and countless e-mails demanding charges. He said he's still wading through the evidence. He declined to answer questions on why the events depicted in the video aren't enough to decide on charges.

"It's a violation of my ethics to talk about and evaluate evidence before a charging decision and I will not do that. I will say this: That that video is graphic and horrific and terrible and no person should do that. But my job in the end is to prove that he violated criminal statute. And there is other evidence that does not support a criminal charge. We need to wade through all of that evidence to come to a meaningful determination and we are doing that to the best of our ability."

Freeman later issued a statement "that it is critical to review all the evidence because at the time of trial, invariably, all that information will be used."

Their comments came one day after Trump made his first comments about Floyd's death. While in Florida for what turned out to be the delayed SpaceX launch, the president called the incident "a very, very sad event" and added that he expected to receive a full report when he returns to the nation's capital.

The federal probe is separate from investigations into Floyd's death by the state Bureau of Criminal Apprehension and the Hennepin County Attorney's Office. In the meantime, calls are growing louder by the hour for the officers to be arrested and charged.

The FBI also is looking for anyone to come forward who might have useful information about the circumstances surrounding Floyd's death. "No tip is too small," said Drolshagen. To submit a tip, call 1-800-CALL-FBI.

The Associated Press contributed to this report.

Andy Mannix • 612-673-4036

Paul Walsh • 612-673-4482