White House Office of Personnel Management (OPM) officials are being threatened with subpoenas if they refuse to testify before the House Oversight and Government Reform Committee, the panel’s chairman said during an interview on C-SPAN’s “Newsmakers.”

The committee on Tuesday is holding the first congressional hearing on the massive digital theft of millions of federal workers’ records.

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“OPM is being very resistant to agree to attend,” Rep. Jason Chaffetz Jason ChaffetzThe myth of the conservative bestseller Elijah Cummings, Democratic chairman and powerful Trump critic, dies at 68 House Oversight panel demands DeVos turn over personal email records MORE (R-Utah) said. “I’m prepared to issue a subpoena if need be to get them there.”

Top officials from both the OPM and the Department of Homeland Security are expected to testify.

“I think the public needs to hear about this; public employees need to hear about this,” he added.

Reports circulated last week that the OPM breach is much worse than initially believed. The Hill reported that up to 14 million people could be affected, far surpassing the 4 million estimate the government first gave.

The Obama administration acknowledged late Friday it had discovered a second hack that exposed millions of background check files on military and intelligence agency workers, many of whom have security clearances.

Chaffetz also indicated in his interview that the suspected Chinese hackers might have been roaming the OPM's networks undetected for up to a full year.

If true, that would triple the timeline initially given by the administration, which said it took four months to discover the hackers.

“The worry is for somewhere between four months and a year, these hackers were in there surfing, being able to extract information, the most sensitive information we have about personal records,” Chaffetz said.

The lawmaker placed blame squarely on the OPM, saying the agency failed to heed the advice of watchdog reports. A November inspector general report highlighted numerous security shortcomings.

“There have been multiple reports that have been out there warning them, telling them that they need to make sure these things are encrypted,” Chaffetz said. “They didn’t do that.”

The largest federal employee union on Thursday criticized the OPM for not encrypting all of its personnel data.

“I don’t think our government took [the warnings] seriously enough,” Chaffetz said.