Federal investigators have accused two former contract employees for Xcel Energy Inc. of willfully violating procedures and falsifying reports about safety-related tests of casks filled with high-level nuclear waste stored at the Monticello, Minn., nuclear power plant.

The findings released Monday by investigators for the U.S. Nuclear Regulatory Commission (NRC) also allege that Xcel officials did not monitor the work of the contract employees as they placed dye on welds to look for cracks in late 2013. The improper testing was discovered by an NRC inspector who checked videos of the work.

After the problem was discovered, Xcel said, the casks were rechecked before most of them were placed in a concrete bunker outside the reactor building. They could remain on the site for years because no permanent national storage site has been built for such waste.

"We are confident the storage canisters are safe and secure, and the welds are good," Laura McCarten, regional vice president for Minneapolis-based Xcel Energy, said in an interview.

McCarten said the company responsible for the improper testing, TriVis Inc., of Birmingham, Ala., is no longer a contractor for the company, and Xcel has taken steps to ensure that the problem doesn't happen again. TriVis could not be reached because its phone number is no longer in service.

David Lochbaum, who directs the nuclear safety project for the Union of Concerned Scientists, said oversight of outside contractors "is a continuing problem in the nuclear industry." He agreed with Xcel that the Monticello casks appear to be safe.

"The people living around the plant and the workers are not risking death because of the problem," Lochbaum said.

Even so, the lapse could have significant consequences for Xcel. As the operator of the reactor, Xcel bears responsibility for overseeing outside companies. Scott Northard, vice president of Xcel nuclear fleet operations, said in the future such tests will be monitored in person by Xcel employees to confirm they are done properly.

The NRC's Office of Investigations, which investigated the Monticello incident, sometimes brings criminal cases with the U.S. Justice Department. The findings released Monday are not criminal charges, but they put Xcel on notice that the NRC is considering "escalated enforcement action." Xcel has the right to respond in writing, at a meeting with the NRC or via arbitration.

Northard said NRC officials told the company that Xcel is not accused of willful violations. He said Xcel does not expect to face criminal action. He said NRC officials would not reveal what enforcement action is being considered against TriVis employees.

Both of the unnamed technicians were testing for cracks on welds sealing the lids on six casks that had been filled with highly radioactive spent nuclear fuel rods. Test procedures called for placing dye on the welds for 10 to 15 minutes and then looking for cracks.

But the resident NRC inspector, and later other investigators, discovered from videos that the workers waited for far less time, as little as 23 seconds. Yet both workers falsified forms, reporting they waited 10 minutes, the investigative findings said.

The technicians said they hadn't read the rules or gotten bad information from a supervisor. One worker told investigators that "he was rushing himself because he had heard contractor management complain about employees working too slowly." The NRC said the technicians' supervisor "apparently was not routinely on site and did not review the videotapes."

Xcel's operation of the Monticello plant has been under scrutiny from regulators. In 2014, an NRC official said Xcel needed to improve after inspectors found lapses in the reactor's flood-response plan. Four months ago, the NRC reported a security-related violation at the plant. Northard said Xcel has corrected those issues and passed reinspections by the NRC.

Separately, the Minnesota Public Utilities Commission this year largely blamed Xcel's "imprudent management" for cost overruns that drove up the price of a five-year Monticello upgrade from $320 million to $748 million. Xcel has defended the upgrade, saying the 1970s-era reactor was largely rebuilt, extending its life and boosting its output.

John LaForge, director of NukeWatch, a Wisconsin-based group that advocates phasing out nuclear power, said the faulty weld testing should never have happened at a large company like Xcel.

"It is an example of cutting corners," he said. "It is a serious violation of the public trust."

NRC policies say the federal agency "cannot tolerate willful violations" because its regulatory program relies on people "acting with integrity and communicating with candor." In a 2013 investigation, a manager at a New York nuclear power plant was arrested by NRC and Justice Department investigators and later pleaded guilty to deliberate misconduct for falsifying test results of diesel fuel needed for backup generators. The worker got 18 months of probation.