U.S. Air Force security guards stationed at Joint Base San Antonio-Lackland exchanged gunfire with an unidentified assailant near the Texas base’s Chapman Training Annex early Saturday morning, according to The Associated Press (AP).

The shooting took place at approximately 4:30 a.m. when a sedan drove past the main gate and opened fire, prompting security guards to immediately returned fire, according to AP.

Stephanie Antosh, the base’s public affairs chief, claimed there was more than one person in the vehicle, the San Antonio Express-News reported. The authorities do not presently know the motive behind the attack at the Air Force base.

“We don’t know what, if anything, started it. But it wasn’t an active threat to the installation, and there is no active threat to the installation,” she said. (RELATED: Military Spouse Opened Fire On Man Who Breached Military Base)

Security guards return fire after drive-by shooting at entrance to Air Force base: officialshttps://t.co/NiM8NPRgDI

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The gate to the Chapman Annex was reportedly temporarily closed in response to the incident and motorists were directed to use alternate routes. The gate was reopened at about 9:40 a.m. and the base continued its operations under a moderate level of security readiness, the San Antonio Express-News reported.

San Antonio police officers also responded to the scene and assisted in the collection of evidence, according to AP.

The incident at the base follows an April 8, 2016 murder-suicide involving an airman, Tech. Sgt. Steven D. Bellino, and his commander, Lt. Col. William A. Schroeder, the U.S. Air Force reported. The base was locked down as military and civilian authorities responded.