In a notice sent to the company, U.S. Department of Transportation regulators alleged a chain of bad decisions by Exxon leading up to the spill and in its immediate aftermath. That included Exxon employees' failure to close an upstream safety valve, which could have significantly reduced the size of the spill after it was first detected.
As a result, the agency said, oil continued gushing into the flooding river for almost an hour after the break was noticed by pipeline controllers in Houston.
The agency also faulted the company for not addressing flood risks and not taking adequate measures to prevent a spill.
The July 2011 rupture of the 12-inch pipeline under the river near Laurel fouled 70 miles of the Yellowstone River's banks, killing fish and wildlife and prompting a massive, months-long cleanup.
Investigators chalked up the immediate cause to floodwaters that damaged the line and left it exposed. It ruptured under pressure from debris washing downriver.
Exxon spokesman Patrick Henretty said the company was disappointed in the government's findings, which he said appeared to contradict an investigative report released in December that said Exxon took "reasonable precautions to address the flooding."
Henretty added that Exxon was still reviewing Monday's notice. He said the Irving, Texas-based company has already altered its training program and procedures on the use of remote-control valves that can be used to shut down pipelines quickly when accidents occur.
Investigators previously said the size of the spill could have been reduced by about two-thirds if pipeline controllers had acted more quickly.
In Monday's notice, the agency said there had been "numerous indications" that the 20-year-old Silvertip pipeline had been installed in an area prone to seasonal flooding and erosion.
Nevertheless, when the Yellowstone was flooding in 2011, Exxon chose to keep its pipeline operating even as at least one other company decided to shut down another line in the same area, the agency said.
Exxon's "failure over an extended period of time to recognize those threats ... was a major cause of the failure," Transportation officials said in Monday's notice.
City officials in Laurel had warned Exxon that the riverbank was eroding. The company, however, continued to run crude beneath the Yellowstone after finding that a section of pipeline leading away from the river was still buried more than 6 feet deep.
The DOT issued a proposed compliance order directing Exxon to conduct training for supervisors and pipeline control room workers, including on the proper operation of remote control valves. The order said Exxon must complete the training within 30 days.
It was not immediately clear whether the training cited by Henretty would fully address the government's proposal.
Exxon has 30 days to appeal or the proposal will become final.
The company has previously acknowledged responsibility for the spill and pledged to work with state and federal officials to make sure the cleanup is adequate.
Exxon spent $135 million on its response to the spill, including cleanup and repair work. The damaged section of pipeline has since been replaced with a new section buried dozens of feet beneath the riverbed.
The accident helped prompt a national debate over the adequacy of federal regulations for the nation's sprawling, 2.6-million-mile network of gas and hazardous liquid pipelines. In Silvertip's aftermath, Congress passed measures intended to make sure pipelines that cross major waterways are sufficiently buried to protect them from the type of damage the Silvertip line sustained.
Under current rules, companies must bury pipelines 4 feet beneath a riverbed and inspect them periodically. Those rules are being reviewed by the Department of Transportation for potential changes.
In a statement, Transportation Secretary Ray LaHood said his agency will hold companies accountable for adhering to federal safety standards.
"It is our priority to ensure that America's transportation system is the safest in the world," LaHood said.
Montana officials are continuing to investigate the natural resource damages caused by the Silvertip spill. Results are expected possibly in the spring.
That process is separate is separate from a $1.6 million settlement reached earlier this year between the state and Exxon over the company's water pollution violations.Suggest a correction