Abdul Hafiz, a terror suspect released from the Guantanamo Bay prison in December, probably has joined the Taliban in Afghanistan, a U.S. counterterrorism official said.

If true, it would be the first known case since President Barack Obama took office of a former detainee at the facility in Cuba joining a terrorist group.

The official, who spoke on condition of anonymity, said Hafiz’s role in the Taliban hasn’t been determined. And an administration official, also speaking on condition of anonymity, said the intelligence on Hafiz’s status remains unclear and incomplete.

The likelihood that Hafiz is with the Taliban was first reported by the Long War Journal, a Mount Laurel, N.J.-based publication that reports on intelligence matters.

In a Feb. 1 letter to House Speaker Nancy Pelosi, White House counterterrorism adviser John Brennan said former detainees suspected or confirmed to have joined militant groups had been set free under President George W. Bush. In his letter, Brennan said 20 percent of freed Guantanamo detainees were known or suspected of having terrorist ties after release.

Congressional Republicans have been trying to get confirmation about Hafiz. If he has joined the Taliban, “it would tell you this is one colossal mistake” to have released him, said Rep. Pete Hoekstra of Michigan, ranking Republican on the House Intelligence Committee. “It typically takes a pretty extended period of time” to confirm such reports, he said.

Hafiz, a citizen of Afghanistan, had been linked to the murder of an International Red Cross worker in Afghanistan and was working with the Taliban, according to a Defense Department memo obtained by the New York Times.