Passengers and crew were stunned when a Delta Airlines pilot was taken into custody by federal agents right after landing a plane in San Francisco.

The arrest happened on Delta Flight 2809 at about 9:35 p.m. on Saturday, July 26. The flight had just arrived from Minneapolis and was getting ready to let passengers off when the pilot was arrested.

Before anyone could leave the plane, around ten federal officers—including agents from Homeland Security Investigations (HSI)—came onboard. They walked straight to the cockpit, handcuffed the co-pilot, and escorted him off the plane, according to one of the passengers who witnessed it.

Passenger Sarah Christianson told the San Francisco Chronicle that agents with badges, guns, and vests from different agencies rushed past the seated passengers to the cockpit.

After removing the pilot, another group of officers returned to collect his personal bags. The rest of the flight crew appeared just as confused. According to Christianson, the other pilot informed passengers that he had no idea what was going on either.

So far, there has been no official explanation about why the pilot was arrested.

HSI, a part of ICE under the Department of Homeland Security, often carries out immigration-related arrests. The agency has been especially active in such operations since the Trump administration launched a broad crackdown on undocumented immigrants.

Christianson described the event as deeply upsetting and strange. She said watching someone be taken away so suddenly and without explanation felt disturbing.

HSI has yet to confirm the reason for the arrest or say if it was related to immigration issues.