The White House and U.S. Capitol were briefly put on lockdown Tuesday morning after authorities said an unauthorized aircraft entered restricted airspace near the Mall.

Officials later said it wasn’t clear what had caused the concern.

An alert was issued about 8:30 a.m. and the buildings were locked down “due to a potential violation of the restricted airspace in the National Capital Region,” according to a spokesman for the U.S. Secret Service.

A U.S. Capitol Police spokeswoman said that “in an abundance of caution … access to Capitol Complex buildings was restricted for a short time.” A D.C. police spokeswoman said the alert involved an aircraft.

North American Aerospace Defense Command officials initially said they scrambled aircraft to respond to a violation of airspace, adding that the aircraft was not considered hostile.

But on Tuesday afternoon, Maj. Andrew Hennessy, a U.S. military spokesman, said no fighter jets were scrambled in the incident.

Hennessy said NORAD dispatched a single Coast Guard helicopter to respond to something flying a “track of interest” over Washington’s Special Flight Rules Area, which restricts air traffic over the region as a security measure. The helicopter crew found nothing of concern and the event was considered resolved without incident, Hennessy said.

The lockdowns were lifted about 9:15 a.m. Hennessy said it wasn’t immediately clear what prompted the initial concern.