MATTAWAMKEAG, Maine — Crude oil continued to flow safely Friday from 13 tanker cars that tipped over when a 96-car Pan Am Railways train derailed Thursday about 300 feet from the Penobscot River, officials said.

Crews from Pan Am, the Maine Department of Environmental Protection and contractor Clean Harbors will work through the weekend. They could finish the work early next week, Pan Am Railways Executive Vice President Cynthia Scarano said.

“Right now it looks as if we are working around the clock,” Scarano said Friday, “but we will be making those evaluations as we continue.”

The tanks do not appear to have ruptured, and no significant environmental damage is apparent. Almost half the oil on scene was pumped to other tankers by Friday afternoon, Maine Department of Environmental Protection spokeswoman Samantha Warren said.

“We are pleased with the process and the progress that is being made,” Warren said. “Four hundred thousand gallons is a lot to move and it could not be going better.”

Rotating three-person DEP teams have been working the cleanup constantly since the accident occurred about 5 a.m. Thursday. Warren said she was optimistic that DEP responders would have the oil safely transferred by Saturday night.

The 13 cars, part of a 15-car string that tipped over, were said to contain 31,000 gallons each. Three gallons of oil that Pan Am officials said typically accumulate around the tanks’ hatches were spilled.

A spokesman for the Federal Railroad Administration said the accident investigation is continuing. It could be a year before it is finished, spokesman Rob Kulat said.

The accident’s occurrence so close to the river spurred renewed discussion at the State House on Thursday about the safety of transporting hazardous materials. Scarano has said that a large-scale environmental problem arising from the accident is unlikely.

Kulat said Friday that the Federal Railroad Administration considers the track in good condition, with inspections done on it at least weekly as federal law requires. Scarano said the tracks are inspected as often as four times a week.

Pan Am replaced most of the ties along the line near the accident last fall to maintain the Class 1 designation and 10 mph speed limit. The work is expected to resume this summer, said Kulat, who said he did not have a detailed description of the project available.

“They still have a little bit of work left to be done,” Kulat said.

A Class 1 rating does not necessarily indicate that tracks are in poor condition, Kulat said.

The speed limit is posted on rails as on roads — according to terrain such as sharp inclines or curves, plus frequency of use and surrounding conditions. Rail yards and downtowns are typical Class 1 areas, he said.

“Not everybody wants to go 60 mph,” Kulat said.