Newly revealed testimony from top military commanders involved in the U.S. response to the Benghazi attacks suggests that the perpetrators of a second, dawn attack on a CIA complex probably were different from those who penetrated the U.S. diplomatic mission the evening before and set it ablaze, killing Ambassador Chris Stevens and another American.

The second attack, which killed two security contractors, showed clear military training, retired 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.

The testimony could help clarify the events of Sept. 11, 2012, that have stirred bitter recriminations in the U.S., including Republican-led congressional investigations and campaign-season denunciations of the Obama administration, which made inaccurate statements about the Libyan attacks.

The testimony underscores a key detail that sometimes has been lost in the debate: that the attacks were two distinct events over two days on two different buildings, perhaps by unrelated groups.

The U.S. government still has not fully characterized the first attack in which, according to Ham and eight other military officers, men who seemed familiar with the lightly protected diplomatic compound breached it and set it on fire, killing Stevens and communications specialist Sean Smith. A disorganized mob of looters then overran the facility.

In testimony to two House panels earlier this year, the military officers said that commanders didn’t have the information they needed to understand the nature of the attack, that they were unaware of the extent of the U.S. presence in Benghazi at the time and that they were convinced erroneously for a time that they were facing a hostage crisis without the ability to move military assets into place that would be of any use.

To this day, it’s not clear if the violence resulted from a well-planned, multiphase military-type assault or from a loosely connected, escalating chain of events.

In their testimony, military officials expressed some uncertainty about the first attack, describing protests and looting in an assault that lasted about 45 minutes.

Ham testified that the second attack, which killed security officers Tyrone Woods and Glen Doherty 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 new team of militants, taking advantage after reports of violence at the first site and American vulnerability.

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,” said Ham, who headed the U.S. command in Africa.

The attacks came as President Barack Obama was in a close re-election battle, campaigning in part on the contention that al-Qaida no longer posed a significant threat to the United States. A terror attack on American assets could have damaged that argument.

Five days after the attack, U.N. Ambassador Susan Rice linked the Benghazi attacks to protests in Tunisia and Cairo over an anti-Islam video. Weeks later, U.S. officials retracted that account but never fully articulated a new one.

Republicans seized on the inaccuracies, contending that the Obama administration was covering up a terror attack for political gain.

Several congressional and independent investigations have faulted the State Department for inadequate security, but they have not provided a full reading of who was involved in the violence, what the motives were and how they could pull off such a multipronged assault.