Evidence suggests that the Benghazi attacks were the work of two separate militant groups, commanders involved in the U.S. response to the attacks told the House Armed Services Committee in testimony released on Wednesday. The deadly 2012 attacks killed four Americans, including U.S. Ambassador Christopher Stevens. 

We already knew that there were two separate waves of attacks on the diplomatic compound: the first attack killed Stevens and a second American at the consulate. But as the Associated Press explains in their advance look at the testimony, a second attack had the markings of a different group of militants. That group killed two American security contractors. The AP explains: 

The second attack in the eastern Libyan city, which killed two American security contractors, showed clear military training, retired U.S. Gen. Carter Ham told Congress in closed-door testimony released late Wednesday. The assault probably was the work of a new team of militants, seizing on reports of violence at the diplomatic mission the night before and hitting the Americans while they were most vulnerable, Ham said.

Ham went on to say in the testimony that the second attack had "a degree of sophistication and military training that is relatively unusual and certainly, I think, indicates that this was not a pickup team. This was not a couple of guys who just found a mortar someplace." Officials also told the committee that they weren't entirely sure what started the first attack, which began with an assault from a group of men who seemed familiar with the premises, before the compound was overtaken by a disorganized mob. 

The Benghazi attacks have been a subject of fierce debate in Congress since 2012, as House Republicans continue to pursue investigations into the attacks. Those investigations are based on a theory popular among conservatives that the Obama administration covered up details of the attack because it contained a politically inconvenient reality. Democrats believe that the testimony released Wednesday complicates that theory. Here's a statement to that effect from the House committee's ranking member Adam Smith, via the Hill

First and foremost, our military did everything it could to save American lives in Benghazi. The military responded appropriately, quickly, to the best of its ability at that time, and no “stand down” order was ever issued. Any suggestions to the contrary are offensive and downright wrong....

...The transcripts tell a tantalizing story of Americans trying to understand what was happening in Benghazi and save the lives of their countrymen. It was a frantic and difficult effort and unfortunately four brave Americans died. But it was not due to a lack of effort or a government conspiracy, as some continue to claim."

In May, the Republican-controlled House created a Select Committee to investigate the attacks. That investigation, which could take months, comes as the U.S. captured and arrested its first suspect connected to the attacks, Ahmed Abu Khattala. As New York magazine notes, however, that arrest combined with the recently release testimony seems to contain a possible contradiction: the Justice Department has accused Khattala of being involved with both waves of the attack.