"We haven't isolated the source of the mystery sheen," said Leslie Pearson, on-scene response coordinator for the agency.

The weathered oil, which surfaced last Friday, has been measured as a patch up to 1.5 miles in length but has mostly appeared as streaks on the water, the department said.

Estimates are that about three barrels of oil leaked on Friday, another barrel on Monday and up to a barrel on Tuesday, Pearson said.

Officials fear that Cook Inlet, a channel that runs south from Anchorage and is the site of Alaska's oldest producing oil and gas wells, may be plagued by a variety of leaky, corroded pipelines.

Production there started in the 1950s, and some of the facilities still in operation date to the 1960s. Many have been abandoned and are not recorded on any maps, Pearson said.

The mystery sheen was spotted near a pipeline right of way used by Unocal, she said. Because of that, it is believed that one of two Unocal pipelines was the source, she said.

Unocal is evaluating an oil line connected to its Dillon platform in the inlet, said Roxanne Sinz, spokeswoman for the company. The line, which dates back to 1966, was plugged two years ago after it sprang a leak.

But a pressure test using fresh water found no leak in that line, Sinz said.

Before then, Unocal had theorized that the leak came from a gas pipeline connected to the same platform, she said, but tests of that line came out negative.

"Where we stand today is we're methodically checking everything," Sinz said. "We're looking at every line that exists out there."

Unocal, the largest operator in Cook Inlet, has more than 100 miles of offshore pipeline in the inlet, plus about 45 miles of onshore gas pipeline. Just how many miles of other pipeline lies beneath Cook Inlet remains unclear.