PITTSBURGH -- The U.S. attorney's office is conducting a criminal investigation into the collapse of the Ashland Oil Co. storage tank that fouled water in four states, according to two Allegheny County officials.

A federal investigative unit, under the direction of U.S. Attorney J. Alan Johnson, was assembled shortly after the Jan. 2 spill to determine whether the oil company violated federal laws, the officials said.

Johnson has held at least two meetings with federal, state and local officials investigating the spill and monitoring its cleanup, said Allegheny County Police Chief Robert Kroner and county Fire Marshal Martin Jacobs.

Johnson declined comment, as did Ashland spokesman Dan Lacy.

The tank collapsed as it was being filled, sending about 770,000 gallons of diesel fuel into the Monongahela and Ohio rivers.

The Occupational Safety and Health Administration levied an $18,540 fine Thursday against Ashland for allegedly exposing workers to a risk of death or serious injury while cutting apart the tank after the collapse.

The company cut apart the damaged tank to remove samples to determine what caused its failure.

OSHA charged that Ashland failed to use a properly calibrated device to monitor the air for explosion hazards during cutting operations and that one worker had to stand atop the collapsed tank "exposed to a fall hazard of approximately nine feet."

Ashland Oil may appeal the citations, company spokesman Roger Schrum said.