Over the weekend images surfaced online of a Hezbollah parade in Qusair, Syria, featuring U.S. armored personnel carriers affixed with antiaircraft guns. The images prompted a flurry of speculation about the vehicles’ origin and whether the group had pilfered the stocks of the U.S.-supplied Lebanese military.

The armored personnel carrier, known as the M113, is one of the United States’ most ubiquitous armored vehicles and has been in service since the 1960s. The tracked semi-rhombus-shaped vehicle comes in numerous variants and can be outfitted to carry troops and artillery; its chassis was even used as the basis for a nuclear-missile carrier. It has appeared in every major U.S. conflict since the Vietnam War and is used by U.S. police departments and dozens of others countries’ militaries around the world.

As a prominent political and military entity in Lebanon, Hezbollah’s possession of the vehicles could support the theory floated by the defense analyst Tobias Schneider, who tweeted that the personnel carriers were probably taken from the Lebanese Armed Forces, a major recipient of U.S. military aid.

Over the summer, the Lebanese military took possession of dozens of pieces of artillery, armored vehicles, semiautomatic grenade launchers and 1,000 tons of ammunition — all worth about $50 million — as part of the United States’ ongoing efforts to bolster the country’s capacity to fight extremists. The shipment, overseen by the Pentagon and the State Department, brought the amount of U.S. military aid sent to Lebanon in 2016 to $221 million, according to U.S. Ambassador Elizabeth H. Richard.

While Lebanese smugglers have helped move weapons and ammunition to opposition groups in Syria, cases of Lebanese military equipment appearing in the conflict have been rare. In a tweet, the Lebanese military denied that the M113s were taken from its stocks, a claim backed up by a State Department official, who spoke on the condition of anonymity to discuss the issue.

“The Lebanese military has publicly stated that the M113s depicted online were never part of their equipment roster,” the official said. “Our initial assessment concurs: The M113s allegedly in Hezbollah’s possession in Syria are unlikely to have come from the Lebanese military. We are working closely with our colleagues in the Pentagon and in the Intelligence Community on to resolve this issue.”

After comparing the “structual analysis of the vehicles in the picture,” Pentagon spokesman Chris Sherwood said that the Pentagon had ruled out the possiblity of Hezbollah taking the M113s from the Lebanese Armed Forces.

Closely aligned with Iran and Syria, Hezbollah has been fighting alongside Syrian government troops since the beginning of the conflict.

The Hezbollah M113s appear to be an older variant, and U.S. officials said they are inclined to believe that vehicles came from the disintegration of the Southern Lebanese Army, or SLA. The SLA was an Israeli-allied and supplied Christian militia that fought during the Lebanese civil war. Its military equipment was ultimately absorbed by Hezbollah in the early 2000s when Israel withdrew from southern Lebanon.

In 1985, Israel supplied 20 M113s to the SLA, according to arms transfer data provided by the Stockholm International Peace Research Institute. From 1984 to 1996, Israel provided more than 130 armored vehicles, tanks and artillery pieces to the SLA, according to the data. Another possibility, as pointed out by Schneider in subsequent tweets, is that Hezbollah took them from Syria’s recently renamed al-Qaeda affiliate, formerly known as Jabhat al-Nusra. It is unclear where al-Nusra got its M113s.

U.S. equipment falling into the hands of extremist groups and regional opponents has been a recurring theme in the Middle East and southwest Asia for the past 15 years as American wares have been distributed wholesale to those willing to fight for U.S. causes. Armored vehicles, weapons, night-vision devices and body armor have been diverted from places such as Iraq, Afghanistan, Syria, Saudi Arabia and Yemen, subsequently showing up on battlefields throughout the region.

The post has been updated to reflect a comment from the Pentagon.