FireEye, a private sector cybersecurity firm, told media that they believe they have discovered who was behind the massive hack on the federal Office of Personnel Management in which millions of federal employees’s data was stolen.

The U.S. Department of Homeland Security alleges that Chinese hackers were behind the attack. Some have gone a step further and claimed that a group affiliated with the Chinese government, called Deep Panda, was behind the attack.

FireEye, however, is asserting that they have evidence they claim shows another group is actually behind the hacking.

“Unlike other actors in China who conduct industrial espionage or steal defense technology, this group has primarily targeted PII [personally identifiable information]. Based on the tools and tactics, FireEye thinks that the group who compromised the OPM’s networks is different from the activity of Deep Panda,” Mike Oppenheim, an intelligence operations manager with FireEye, said in an interview.

FireEye director of threat intelligence Laura Galante was asked if they think the Chinese government was at all involved, and she responded, “That is an open question and we haven’t worked it out at this point.”

The security firm first started monitoring this group in 2013 but has yet to find a name for it. This means that much of what this group does remains a mystery, including how closely affiliated the group is with the Chinese government.

CNN reports that over 18 million employees’s data has been stolen. The Washington Post reports that much of that data was highly sensitive, and the information can be used to target current and former federal employees.

Some statements from senior officials have led some to believe that the Chinese government was somehow involved.

“This was classic espionage, just on a scale we’ve never seen before from a traditional adversary,” one high ranking member of the administration said.

However, the Obama administration has been reluctant to officially blame any particular group or organization for the attack. FBI officials have simply said their investigation is ongoing.

Congress has called OPM director Katherine Archuleta before three committees to testify on the recent discovery of the hacks.

From various congressmen’s statements, they do not seem pleased with her or her agency.

“We need to be more forthcoming with our employees,” Massachusetts Democrat Rep. Stephen Lynch said.

Sen. Ron Johnson, a Republican from Wisconsin, will head a congressional panel on the breach Archuleta is testifying before.

“We need people in place in these [high-ranking positions] that… take the threat of cybersecurity seriously,” he said. “I don’t think the [director] did that.”