An engineering firm accused of overbilling the U.S. Air Force and withholding that information for years agreed Thursday to pay $6.4 million in a settlement with the U.S. Department of Justice on Thursday.

CH2M Hill, a engineering consulting firm that frequently works with the federal government, was accused of overbilling the Air Force more than $8 million for services by underqualified consulting staff and over $2 million in interest between 2003 and 2014. The Justice Department said CH2M Hill knew about overbilling its clients as early as 2011 but would not release a third-party audit of its labor practices, calling the report “a confidential matter,” according to court documents.

“We rely on those who apply for, and receive, government contracts to fulfill their end of the deal — and that includes making sure that the personnel on the job are qualified for the job,” said U.S. Attorney Brian Moran.

According to the settlement, at least five members of CH2M Hill’s consulting staff didn’t meet the educational or labor requirements of two Air Force Center for Engineering and the Environment contracts. The consulting firm, based in Colorado, has an office in Bellevue. It was also in charge of a cleanup project at the Hanford Site from 1999 to 2008, years after the federal government produced plutonium for nuclear weapons during the Cold War.

After a judge ruled the firm had to release its audit in 2016, CH2M Hill paid back the U.S. government in 2017 after acknowledging the Air Force had overpaid it for services.

However, “the government, in our role as trying to deter people not admitting [to] overbilling…is able to seek double damages,” said Emily Langlie, communications director for the U.S. Attorney’s Office. “The overpayment was already returned to the government, but in this settlement they’re paying additional money in resolving this matter.”

The settlement allows the government to collect double the overpayment — but that is limited to $6.4 million because under a 10-year statute of limitations, it could not collect damages from overpayments from 2003 to a portion of 2006.

This isn’t the first time CH2M Hill faced controversy related to federal work in Washington state. In 2013, a handful of managers and supervisors were indicted by a federal grand jury for enabling time fraud when the company was in charge of cleaning up radioactive waste from the Hanford Site in Benton County. The U.S. imposed or collected more than $19 million in fines, damages and penalties.