The Taliban member who rammed an explosives-laden motorcycle into a joint NATO-Afghan patrol near Bagram Air Base in southern Afghanistan Monday, killing six U.S. troops, passed through Afghan security before carrying out the suicide bomb attack, CNN reports.

Two U.S. service members, an American contractor and an Afghan, were also reportedly injured in the attack.

CNN learned from an unnamed U.S. official familiar with the latest information about the incident that “The attack happened after the motorcycle driver passed through an initial perimeter of Afghan security.”

“It appears, according to the U.S. official, that the Afghans did not detect the bike itself was internally packed with explosives. It then approached the troops who were outside and detonated,” notes CNN.

A Pentagon report released last week revealed that the Afghan National Defense and Security Forces (ANDSF) “performance over the entire fighting season and the last six months has been uneven and mixed.”

“The ANDSF have demonstrated a growing capability to plan and execute large-scale offensive operations while, as expected, significant challenges remain in the areas of ANDSF leadership, combat enablers, logistics and sustainment, and ministerial capacity,” added the report.

The Pentagon acknowledged that the ANDSF have suffered record casualties this year as they combat Taliban rebels largely on their own.

“In their first fighting season against an Afghan-led counterinsurgency, the Taliban-led insurgent threat remains resilient,” reported the Pentagon.

By packing explosives inside an old motorcycle, rather than wearing a suicide vest, the Taliban suicide bomber who carried out the terrorist attack Monday was likely able to inflict catastrophic damage, CNN learned from the U.S. official.

“The U.S. troops were on a security patrol outside Bagram Airfield,” reports CNN. “Typically, patrol missions require troops to walk with space between individuals in order to prevent mass casualties in the event of a suicide attack.”

“Because the Americans were walking in a narrow lane surrounded by walls, the blast bounced off the walls, rather than dispersing, and the power was contained in a relatively small area,” it adds. “The bomb material was small metal balls that dispersed at a high velocity in that contained area, the official said.”

Four of the U.S. airmen were members of the Air Force Office of Special Investigations.

CNN quoted a senior military official as saying that “it is not unusual for such personnel to undertake security patrols. The patrol had been sent out to talk to local Afghans outside Bagram in part because there are several villages so close to the base, a major area for U.S. military operations. The patrols are also conducted so U.S. personnel can gather intelligence to keep rockets and mortars from being fired at the base.”

The remaining two U.S. troops, which include 15 year-veteran of the New York Police Department Technical Sgt. Joseph G. Lemm, were assigned to the Security Forces Squadron at Stewart Air National Guard Base in New York.

CNN reports, “The bodies of the six Americans were flown home Wednesday. Their flag-draped caskets arrived at New Castle Air National Guard Base in Delaware for dignified transfer.”

According to the Pentagon, killed in the attack were:

Taliban-related violence has increased since President Obama and NATO ended their combat mission in December 2014 and transitioned to a train, advise, and assist (TAA) role. Most U.S. and international troops have been withdrawn from the war-torn country.

“The Taliban-led insurgency has likely been emboldened by the coalition’s transition from direct combat operations to a TAA role and the accompanying reduction of coalition combat enablers,” notes the Pentagon report. “As a result, the Taliban will continue to test the ANDSF aggressively in 2016.”