Moments before a natural gas leak at Minnehaha Academy’s upper campus ignited on Aug. 2, triggering an explosion that killed two people and devastated part of the south Minneapolis school, a maintenance worker, sensing danger, desperately tried to radio instructions to clear the building.

These were among new details about the explosion to emerge in a preliminary report released Monday afternoon by the National Transportation Safety Board (NTSB), the primary investigative agency in the case. The report did not identify the school worker.

Two staff members were killed and nine others were injured in the blast, which tore a hole through the central portion of the building near E. Lake Street and West River Parkway. Because of summer break, relatively few people were in the building at the time.

Had school been in session, authorities say, casualties almost certainly would’ve been higher.

Safety investigators have said they were focusing on the source of the leak that caused the explosion. One possible explanation floated by the federal agency earlier this month was that two contractors, a father and son from Master Mechanical in Eagan, were in the process of moving a gas meter from inside the school to outside when the leak occurred.

Authorities haven’t said whether the pipe that carries natural gas from a main under West River Parkway into the school was shut off during what one NTSB official termed a “very hazardous operation.”

“At the time of the explosion, two workers were installing new piping to support the relocation of gas meters from the basement of the building to the outside,” investigators said in the two-page report released Monday afternoon. “Two new meters mounted on a wall were ready for the new piping to be connected.”

The report continued: “While workers were removing the existing piping, a full-flow natural gas line at pressure was opened.”

Unable to contain the leak, the two workers left the building, investigators said. It wasn’t immediately clear whether they alerted school officials on their way out.

A short time later, the unnamed maintenance worker followed the smell and sound of escaping gas and went down to the basement to investigate. Natural gas is odorless, but a substance called mercaptan is often added to it, giving it a distinctive rotten-egg scent to help detect leaks.

After realizing the danger, the worker got on his walkie talkie to tell other staff members to evacuate the building, before racing upstairs to warn others, the report said. How many people his frantic message reached is unclear.

Less than a minute later, the building exploded, the report said.

Officials cautioned that the findings released Monday were preliminary. A final report may not be released for several months.

For several days after the blast, the NTSB investigation was hampered by concerns about the building’s structural integrity.

The contractors who were working inside the school, neither of whom has been identified, have spoken with investigators, NTSB officials said.

City records show Master Mechanical had previously done work at the school, in 2003 and 2004.

NTSB officials didn’t respond to a follow-up e-mail on Monday afternoon, and a spokesman for the city’s fire department said he hadn’t yet seen the report.

Killed in the blast were receptionist Ruth Berg, 47, and part-time custodian John F. Carlson, 82, who also worked as a Metro Transit bus driver for more than three decades. At the time, officials said the two bodies were found near each other, both on the south side of the collapsed portion of the building.

Nine other people were wounded, including Bryan Duffey, a full-time custodian, who was also in the area when the blast occurred and was severely injured. It is not known whether Duffey was the maintenance worker in the NTSB report.

Investigators have said their probe aims to answer such questions as whether protocols were properly followed. They will also seek to determine the training and mind-sets of the two contractors, including whether they were tired, distracted or otherwise impaired on the day of the blast.