ManTech International Corp., a Fairfax government contractor, has fired several employees after an internal probe revealed that documents were altered before being turned over for a Defense Department investigation.

"A limited number have been terminated, and the investigation is ongoing," said Peter Lamante, a company spokesman. Lamante declined to say how many employees had been fired or whether more dismissals could be coming.

The probe centers on a multiyear contract ManTech held with the Defense Security Service, which issues security clearances and conducts background checks. Under the contract, which ended earlier this year, ManTech employees helped conduct the background checks, but did not handle classified information, said Lamante.

In October, the Defense Department inspector general's office opened an investigation into whether ManTech had overcharged and if all employees had the required security clearances. After receiving the subpoena, ManTech launched an internal investigation, Lamante said.

The company discovered that five time sheets for one employee, who did not possess the proper security clearances, had been changed before being turned over to the Defense Department. The time sheets were changed to show that the government had not paid the employees' salary, according to Securities and Exchange Commission filings.

"We promptly informed the government of this development, and we terminated certain employees" of a subsidiary after the discovery, the filing said. A Defense Department spokesman declined to comment.

ManTech is also the subject of an Environmental Protection Agency probe. The agency is investigating whether the company charged it for more hours than it worked under the contract, according to an SEC filing. A federal grand jury is also investigating whether ManTech improperly charged the government for the cost of an acquisition, according to the filing. ManTech received the first subpoena last year and another on Aug. 2.

"Government inquiries are a regular part of the business of contracting with the government," Lamante said. "We are cooperating fully, and none of the current investigations we believe will have a material adverse affect on our financial performance."

The disclosure of the Defense Department probe coincided with an announcement that ManTech would be selling more than 6 million shares to the public, raising money for acquisitions but diluting shareholder interest. The company's stock lost 15 percent that day, closing at $18.80. It has since regained most of that ground, losing 18 cents yesterday to close at $20.