Are two sets of terrorists responsible for the Benghazi attacks?

Well-trained attackers executed the 2012 deadly dawn assault on a CIA complex in Benghazi, Libya, suggesting different perpetrators from those who penetrated the U.S. diplomatic mission the previous night, according to newly revealed testimony from top military commanders.

The initial attack on Sept. 11, 2012, killed U.S. Ambassador Chris Stevens and communications specialist Sean Smith and set the mission ablaze. Nearly eight hours later at the nearby CIA complex, American contract security officers Tyrone Woods and Glen Doherty died in a mortar attack that showed clear military training, retired Gen. Carter Ham told Congress in closed-door testimony earlier this year. The House Armed Services Committee released the testimony Wednesday.

Two House panels—Armed Services and Oversight and Government Reform—conducted interviews with nine senior officers between January and April. In their testimony, military officials expressed some uncertainty about the first attack, describing protests and looting in an assault that lasted about 45 minutes.

The military attache to the U.S. Embassy in Tripoli told Congress the first attack showed some advance planning. The Libyan police officer guarding the diplomatic compound fled as it began. The defense attache, whose name wasn’t released, suggested the attackers “had something on the shelf”—an outline of a plan based on previously obtained information about the compound and its security measures. They were ready to strike when the opportunity arose.

But Ham testified the second attack, at the annex a mile from the diplomatic compound where the assault began the night before, showed clear military training. It was probably the work of a different team of militants, taking advantage of American vulnerability after reports of violence at the first site.

“Given the precision of the attack, it was a well-trained mortar crew, and in my estimation they probably had a well-trained observer,” said Ham, who headed the U.S. Command in Africa. The second attack showed “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.”

Bitter recriminations in the United States followed the 2012 attacks, including Republican-led congressional investigations and campaign-season denunciations of the Obama administration, which made inaccurate statements about the incidents. The testimony released Wednesday underscored a key detail sometimes lost in the debate: The attacks were two distinct events over two days on two different buildings, perhaps by unrelated groups. To this day, despite the investigations, it’s not clear whether the violence resulted from a well-planned, multiphase, military-type assault or from a loosely connected, escalating chain of events.

Many of the military officials said they didn’t even know about the diplomatic mission in Benghazi, let alone the CIA’s clandestine installation nearby. Few knew of Stevens visiting the city that day. Given all the confusion, Ham said there was one thing he clearly would have done differently: “Advise the ambassador to not go to Benghazi.”

The Associated Press contributed to this report.