The explosion that killed 37 people and injured more than 100 others last week at the Petroleos Mexicanos, or Pemex, headquarters in Mexico City was caused by an accumulation of gas, officials said.

"The cause is clear. It was a gas explosion," Attorney General Jesus Murillo said, citing investigators' conclusions.

The blast was not caused by a bomb because laboratory tests found no traces of explosives, the AG said.

The explosion was likely caused by an accumulation of methane gas ignited by a spark, Murillo said.

Investigators, however, have not determined where the gas leak occurred in the building's basement, the AG said.

Workers were doing maintenance on the structure's support columns when the blast happened.

"Several sources are possible" for the gas, Murillo said.

Investigators will continue trying to determine who was responsible if negligence was involved, the AG said.

Pemex's headquarters was torn by a blast last Thursday that occurred in the basement of one of the complex's buildings.

The main tower of the office complex, a 56-story structure, was not damaged in the blast. EFE