SEATTLE -- State regulators on Tuesday released their investigation into the natural gas explosion that rocked Seattle's Greenwood neighborhood last March. Investigators say the blast was caused by a broken gas line that should have been capped off years ago and are filing a complaint against Puget Sound Energy for 17 safety violations and a recommended $3.2 million penalty.

The March 9 blast injured nine firefighters and caused extensive property damage to several Greenwood businesses.

According to Washington Utilities and Transportation Commission’s Pipeline Safety staff, which led the investigation, the explosion was caused by outside force applied by "unauthorized individuals" to a Puget Sound Energy gas service line. That damage allowed gas to escape, accumulate in the building occupied by Mr. Gyros restaurant, and ultimately explode.

UTC investigators say the broken gas service line was one that PSE records showed as abandoned in 2004, but PSE failed to properly disconnect and seal the line, "allowing it to remain in service for nearly 12 years without proper oversight."

"The gas service line that PSE believed was abandoned in 2004 was located in a space not intended for human occupancy between the Mr. Gyros and Neptune Coffee businesses. Evidence at the scene and witness interviews indicated that the space around the gas line was frequently used by unauthorized individuals to store personal items. Individuals acknowledged to investigators that due to the limited space, they sometimes disturbed the gas line to access the area," Washington UTC said in a released statement.

As a result of the investigation, the UTC filed a formal complaint against PSE, alleging the company committed 17 violations of pipeline safety regulations and recommended a penalty of up to $3.2 million.

The complaint will be scheduled for a hearing before the three-member commission.

Read full statement by Washington Utilities and Transportation Commission

PHOTOS: Massive explosion in Seattle's Greenwood neighborhood

At an evening meeting Tuesday, property owner Mike Slattery told the Greenwood Council that the insurance company has not been cooperative. He said the report calling for PSE to be fined is welcome news.

“With the partial finger being pointed to PSE, it strengthens our case,” said Slattery.

Business owners hope it will clear the way for insurance settlements.

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