Investigation finds electrical fire that temporarily closed State House was accidental

Investigators determined that Tuesday's electrical fire in the State House's building, which forced hundreds of lawmakers, employees and visitors to evacuate, was accidental.

No one was injured in the incident, which first responders said generated significant volumes of carbon monoxide and required power to be shut down. The building remained closed Wednesday, according to an announcement by the Bureau of the State House Tuesday evening.

A joint investigation found that "the fire resulted from electrical faults in two lines that run from an electrical vault near a guard shack to a pull box in the sub-basement of the Annex Building," according to a statement by the Massachusetts State Police.

Firefighters responded to the capitol shortly after 2 p.m. Tuesday and found "a lot of smoke coming from the vault area" in the basement, Boston Fire Department Commissioner Paul Burke told reporters around 4:45 p.m. They determined that high-voltage wires that feed into a transformer, but not the transformer itself, were ablaze.

Electrical equipment was damaged and the basement sustained some smoke damage, but there was no other structural damage, Burke said.

Next steps still were not entirely clear when Burke addressed an assembly of cameras at the corner of Myrtle and Hancock streets.

"The power company is going to evaluate. Once the burning is stopped and it's cooled down, they'll evaluate how to circumvent that area, whenever they can. They could probably fire up two [high-voltage feeds], maybe, and have enough service, but that's up to them," he said.

"It could have been the demand because of the air conditioning, it could have been anything," he said.

First responders spent some time on the two-alarm scene, in hazy July weather, waiting for Eversource to isolate the affected transformer by shutting down power.  State Police first needed to vent the basement before the utility company could  "localize and shut off the feed that is believed to have caused the fire," according to State Police spokesman David Procopio.

To extinguish the fire, fire responders went with "security rope," in case they need to exit quickly. Burke said they were working in a tunnel and toxins coming the burnt material were "very dangerous."

"For carbon monoxide, an average reading might be 10 or 11 parts per million. This was in the thousands. So it's not sustainable for humans to not be on oxygen, on air that we have the firefighters on, so they had to go down there with their equipment on."

The evacuation was in fact the second of the day. On Tuesday morning, the building briefly emptied out after two people accidentally activated an emergency alarm they assumed would open a door, according to investigators. The incident disrupted committee hearings, but people people were allowed back inside after around 15 minutes.

When the second alarm of the day went off, lawmakers, government officials, staffers and members of the public — many of whom seemed to think it would be another brief interruption — mulled about until state troopers began to shepherd them from the area around the State House, saying "This isn't a drill."

State Police said the two incidents were not connected.

The building closure occurred as Democrats continue to haggle over details of an overdue annual state budget, amidst an effort in the House to push a gun law reform bill to the floor, and as cities and towns wait for Democrats to end their stalemate on a road and bridge funding bill.

Senate Minority Leader Bruce Tarr said he's optimistic lawmakers will be able to "absorb" the disruption without too much of an impact.

"We may have to quicken the pace on a couple of things, but I don't think it will be incredibly disruptive unless we find out that there's damage that prevents the building from reopening in the next day or so," the Gloucester Republican told reporters.

The House had an informal session planned for Wednesday at 11 a.m., but top Democrats scrapped that following the announcement the building will remain closed and now intend to reconvene in an informal session on Thursday. Speaker Ron Mariano's office continued to advise representatives of a potential formal session Friday.

This story was reported by the State House New Service's Chris Lisinski, Colin A. Young, Michael P. Norton, Sam Drysdale, Alison Kuznitz and Sam Doran. With reporting from WBUR's Ally Jarmanning.