Updated 12:44 p.m. ET.
The top U.S. workplace safety agency on Thursday imposed $16.6 million in fines against companies involved in a power plant blast that killed six workers and injured 50 others.
The companies "blatantly disregarded well-known and accepted industry procedures and their own safety guidelines" during the work at the Kleen Energy Systems plant in Middletown, the U.S. Occupational Safety and Health Administration said.
OSHA's fines stem from 371 alleged safety and workplace violations identified by its investigators.
The plant exploded Feb. 7 when something ignited natural gas and air that had accumulated in tight quarters as workers cleaned pipes in a procedure known as a "gas blow," which uses natural gas to scrub debris from pipes.
OSHA officials said the fines are the largest in New England in recent history and among the largest nationwide in workplace safety investigations.
The largest proposed fine in the case is $8.3 million against O&G Industries, the general contractor. Seventeen other companies also face potential fines, ranging from $7,000 against several small contractors to more than $6 million against Keystone Construction & Maintenance, which oversaw the gas blow procedure.
The companies have 15 days to pay or contest the fines.
More on the deadly power plant explosion:
Dangerous Gas Buildup Preceded Ct. Blast
Workers: Grueling Shifts at Ct. Blast Site
Official: All Accounted for in Conn. Blast
Uncertainty Hinders Conn. Blast Search
Probe Begins in Deadly Conn. Plant Blast
Five Killed, 12 Hurt in Conn. Explosion