WASHINGTON — A cyberattack similar to previous hacker intrusions from China penetrated computer networks for months at USIS, the government’s leading security clearance contractor, before the company noticed, officials and others familiar with an FBI investigation and related official inquiries told The Associated Press.

The breach, first revealed by the company and government agencies in August, compromised the private records of at least 25,000 employees at the Homeland Security Department and cost the company hundreds of millions of dollars in lost government contracts.

In addition to trying to identify the perpetrators and evaluate the scale of the stolen material, the government inquiries have prompted concerns about why computer detection alarms inside the company failed to quickly notice the hackers and whether federal agencies that hired the company should have monitored its practices more closely.

Former employees of the firm, U.S. Investigations Services LLC, also have raised questions about why the company and the government failed to ensure that outdated background reports containing personal data weren’t regularly purged from the company’s computers.

Details about the investigation and related inquiries were described by federal officials and others familiar with the case. The officials spoke only on condition of anonymity because they were not authorized to comment publicly on the continuing criminal investigation, the others because of concerns about possible litigation.