"It's still in the general investigation phase," said Mark MacIntyre, spokesman for the EPA's Region X office in Seattle.

Alaska's Department of Environmental Conservation launched a probe shortly after the spill to discover the cause behind the leak. It is not a criminal investigation, officials said.

The Wall Street Journal reported Thursday that the EPA has launched a criminal investigation into BP's management of the pipeline corrosion blamed for the spill of an estimated 200,000 gallons of crude oil in the western part of the largest US oil field.

Alaska officials blamed internal corrosion for creating a hole that sprung the oil leak and said crude poured out for several days.

The resulting crude oil spill, the largest ever recorded on Alaska's North Slope, spread over nearly two acres of snow-covered tundra and frozen lake surface.

A spokesman for BP's unit in Alaska said the company does not know if it is the target of a criminal investigation.

"EPA, as well as (Alaska's Department of Environmental Conservation), had investigators on the Slope for the first several days of the spill response," said BP spokesman Daren Beaudo. "We've not had any additional inquiries from EPA."

BP is likely to face a state fine once a final report from Alaska investigators determines a final cause, according to state officials.

Aside from the EPA, BP has been targeted by the US Department of Transportation's Office of Pipeline Safety.

Last month, the Office of Pipeline Safety invoked a little-used provision of law and issued a corrective action order against BP for the leaky pipeline and two other similar pipelines.

The Office of Pipeline Safety order mandates that BP conduct thorough investigations and repairs of the western Prudhoe Bay pipeline that caused the spill, and report back to the federal agency, before putting the line back into service.

The order also mandates inspections and reports of crude oil transmission lines in eastern Prudhoe Bay and at the Lisburne oil field, both about 30-year old lines.