MINNEAPOLIS — Just before an explosion at Minnehaha Academy killed a receptionist and custodian and hurt nine others earlier this month, a maintenance worker heard and smelled gas being released.

He went to the source of the gas, in the basement of the building on the school’s Upper Campus. As he left the basement, he used his hand-held radio to announce the presence of gas and tell everyone to evacuate.

The worker, whom a school spokesperson identified as Don DuBois, then ran up the stairs to search for anyone left inside.

Less than a minute later, the building exploded.

These new details come from a preliminary report released Tuesday by the National Transportation Safety Board, which is investigating the Aug. 2 blast.

NTSB notes that the report will be supplemented or corrected as needed while the investigation continues. The report does not make any conclusions about the explosion.

The report also says that at the time of the explosion, two workers were installing new piping as part of a project to relocate gas meters from the basement to the outside of the building.

An NTSB official has identified the company on the project as Master Mechanical, based in Eagan, Minn.