ENCINITAS - All local beaches were open again Monday, several days after a diesel fuel spill fouled the water off the coast of Encinitas and forced authorities to close roughly two miles of shoreline.

The U.S. Coast Guard is still investigating the source of the spill - estimated at between 500 and 1,000 gallons - that was first reported late Thursday morning by a private boater about four to five miles off the Encinitas coast, said Lt. J.G. Lis Bosma on Monday.

Federal investigators have secured a fuel sample from one ship that passed by the area between midnight Tuesday and 11 a.m. Thursday. Samples from three more vessels were being collected Monday, Bosma said.

"(The suspect ships) were identified by going back through vessel movement at our Vessel Tracking System and there were the four vessels during that time period," Bosma said.

The chemical makeup of fuel is unique to each vessel, Bosma said.

As fuel sits in the tank of a ship, it collects traces of various metals to create a unique "chemical fingerprint," authorities have said. That fingerprint is a highly reliable way of identifying the ship from which the fuel came.

A chemical analysis of the fuel samples gathered from the ships will be compared to fuel gathered at the spill site to try to identify a match. The samples will be analyzed by the Coast Guard's Marine Safety Lab in Groton, Conn., and results should be available in three to five days, Bosma said.

Bosma declined further comment on the investigation.

After the spill was detected, authorities closed a two-mile stretch of beach from Grandview Avenue south to D Street on Thursday. A multi-agency clean-up crew sopped up much of the fuel Friday.

- Contact Philip K. Ireland at (760) 901-4043 or on-line at pireland@nctimes.com.