HARTFORD, Conn. — The nation's top workplace safety agency is imposing $16.6 million in fines against companies involved in a deadly Connecticut power plant blast.

The U.S. Occupational Safety and Health Administration says the fines stem from 371 alleged safety and workplace violations at Kleen Energy Systems, the Middletown power plant that exploded Feb. 7.

The largest proposed fine is $8.3 million against O&G Industries, the main contractor at the plant.

The plant exploded when something ignited natural gas and air that had accumulated in tight quarters as workers cleaned pipes. Six workers died.

The companies have 15 days to pay or contest the fines.