PORTLAND, Ore. -- Federal investigators on Thursday blamed Union Pacific Railroad for a fiery oil-train derailment along the Oregon-Washington border, saying the company failed to maintain its track properly.

Preliminary findings on the June 3 derailment in the Columbia River Gorge raise questions about why the company didn't find the broken bolts that triggered the wreck when it inspected the tracks right before the derailment.

Union Pacific faces potential penalties for safety violations, officials said.

The Associated Press obtained the findings ahead of their Thursday release.

Advanced electronic brakes proposed by regulators could have made the derailment less severe, Federal Railroad Administrator Sarah Feinberg said.

The brakes could have reduced the number of cars that went off the tracks and prevented the one that first burst into flames from being punctured, officials said.

"We're talking about upgrading a brake system that is from the Civil War era," Feinberg said. "It's not too much to ask these companies to improve their braking systems in the event of an accident so fewer cars are derailing."

The railroad industry, through the Association of American Railroads, has lobbied against new braking requirements, saying they would provide "minimal safety benefits" at a price tag topping $3 billion.

Federal officials say putting new brakes just on locomotives hauling large loads of oil and other flammable liquids would cost far less, around $80 million.

The derailment released 42,000 gallons of crude oil and sparked a massive fire that burned for 14 hours.

The mayor of Mosier, the tiny river town where the train derailed, reacted with alarm to the preliminary findings.

Union Pacific said Wednesday it would resume running oil trains through the gorge sometime later this week despite objections from residents and elected officials.

"Where else are there rusty bolts that could fail?" Mayor Arlene Burns said. "I think it's outrageous that they're telling us it's safe."

Union Pacific said its inspections didn't detect the broken bolts that caused the derailment.

The company tests for sideways movement in the rails using a machine that puts an amount of pressure equivalent to a locomotive on the rails -- a stress test that would reveal loose or broken bolts, said Justin Jacobs, a Union Pacific spokesman.

The stress test previously was conducted every 18 months but now will be done every three months, Jacobs said.

Union Pacific also has expanded inspections of all its tracks in Oregon and Washington state, particularly in curved portions of track, he said.

The company has said the rails were inspected as recently as May 31, but it wasn't clear whether the inspection included the stress test. Jacobs didn't know.

The government's preliminary report said walking inspections along the rails were critical to detect broken bolts and track movement.

Feinberg said it was the railroad's responsibility to detect those problems.

"We feel like it could have been prevented with closer inspections, better maintenance," she said.

At least 27 oil trains have been involved in major derailments, fires or oil spills in the U.S. and Canada during the past decade, according to an AP analysis of accident records.

Federal officials have responded with requirements for tougher tank cars, speed restrictions and other actions, but the accidents have continued.

Steven Ditmeyer, a rail consultant who worked in leadership positions at the Federal Railroad Administration and at Burlington Northern Railroad, said sheared-off bolts like those seen in the Oregon derailment are likely symptoms of a larger problem.

In the 1990s, he said, Burlington Northern had a series of derailments on tracks in Washington state that involved sheared-off railroad spikes, which served the same function as the bolts in question in the recent wreck.

Investigators eventually determined overloaded cars, a lack of lubrication between the wheel assembly and the body of the tanker cars and not enough gravel in the railroad bed combined to put strain on the track when the train was in a curve.

That strain loosened the bolts holding the rails to the rail ties and pushed the rails apart, causing derailments, he said.

"When they said those sheared lag bolts, that was the hint that said they really need to look in the direction of these other aspects," Ditmeyer said.

"These are heavy cars when they're fully loaded," and a treatment to reduce the volatility of the Bakken crude makes the oil heavier, he said.