NEW YORK (MarketWatch) — A Coast Guard cutter on Sunday was returning from an area in the Gulf of Mexico, not far from the site of last year’s Deepwater Horizon oil-rig explosion, where a three-mile-long sheen had been spotted Saturday.

A Coast Guard helicopter and crew on Saturday confirmed there was a substance on the water’s surface, and the agency should know more later Sunday after the substance was analyzed, the Coast Guard said.

A Coast Guard vessel obtained samples to determine “what is actually in the water,” Casey Ranel, a spokeswoman for the agency, said Sunday.

The substance was sighted about 20 miles north of the site of the April 20, 2010, BP PLC BP, -1.78% explosion and oil spill that flowed for three months, but the Coast Guard cautioned against jumping to any conclusions.

Oil spill panel blames BP, industry

“There is a lot of dredging in the area, and a bunch of silt below the water level” in the water right at the mouth of the Mississippi River, which could cause the appearance of oil or an oil sheen, said Ranel.

The Coast Guard said it was notified of a three-mile-long, rainbow-colored sheen off the Louisiana coast Saturday morning. Two later sightings were also reported, the last of which had the sheen extending from six miles off the coast of Grand Isle to 100 miles out into the Gulf of Mexico, the Coast Guard said.

Insider trading?

Eleven months after the BP rig blast that killed 11 rig workers, federal investigators are looking into what BP officials knew about the rate of oil spilling into the ocean in the days just after the April 20 blowout, according to the Times-Picayune.

BP officials initially said just 1,000 barrels a day were spewing from the damaged well and then later revised that estimate to 5,000 barrels, but the New Orleans-based newspaper said congressional investigators have found documents indicating BP knew as much as 14,000 barrels could be spilling each day. Government scientists eventually determined the actual spill at 62,000 barrels a day.

The newspaper cited two sources familiar with the Department of Justice probe in saying the investigation is looking at whether BP executives used their knowledge for illegal insider trading.

Last April’s massive oil spill prompted the Obama administration to temporarily suspend deepwater oil and gas drilling in the Gulf of Mexico. Since the moratorium was lifted in October, federal regulators have granted three permits for deepwater drilling in the gulf.