Yale received three subpoenas last week from the U.S. Department of Health and Human Services, Defense Department and National Science Foundation seeking grant documents dating back as far as 10 years.

The subpoenas cover 47 grants valued at about $45 million, the school said. The university received about $2 billion in grants during the past decade.

The school has acknowledged problems with its accounting procedures. In an email to faculty and staff on Friday, Yale President Richard Levin urged employees to cooperate with investigators.

"Regardless of the outcome of the current investigation, we must get all our processes right and make sure that we are good stewards of the funds entrusted to us by the federal government," Levin said in a statement released Monday.

Like most large research universities, Yale relies heavily on government grants to pay for scientific research. The grants come with stringent accounting rules that in one recent case Yale did not follow, federal officials said.

The three agencies issuing subpoenas funded about 90 percent of Yale's research in recent years, the school said. The agencies had no immediate comment Monday.

Such investigations could lead to criminal charges, civil penalties or administrative sanctions.

A report by the inspector general of the Department of Health and Human Services noted in February that university procedures were not always followed or were inadequate.

That report, which focused on a grant to research gene expression, found that more than a third of the university's $508,000 in invoices did not comply with federal standards. Costs were improperly transferred across budget cycles to make up for shortfalls, the report said.

During the review, Yale told researchers that some documents had been altered and said it was investigating.

The report also criticized the school for its oversight of researchers. The primary researcher worked less than promised on the grant application and auditors said the school did not have a process to ensure its researchers were working.

Yale responded to the inquiry by outlining steps it was taking to improve accounting, including hiring a consulting group to speed accounting improvements and hiring staff to monitor its research financing.

-- MATT APUZZO (Associated Press Writer)