Terminal four at Sky Harbor International Airport was evacuated early on Sunday morning as police were brought in to check an unattended vehicle in one of the airport parking lots.

The unattended vehicle was located on level two of the south east side of the terminal.

Terminal four operations returned to normal around 11:15 a.m.

Passengers reported on social media that they had heard explosions in the area. It was unclear if the explosions heard were confirmed by bomb squad personnel, who were on the scene.

According to the Sky Harbor Twitter account, the east side of the pre-security area of terminal four was evacuated as three of the four check points at the terminal were closed.

Phoenix Sky Train has also halted service to terminal four as police worked to resolve the issue.

Officials at Sky Harbor have asked that travelers avoid terminal four until the situation is resolved.

“We’re in the process of making sure this is as smooth as possible to get things up and running again when we are finished with our investigation,” a spokesperson for Phoenix Police said.

Airlines including Air Canada, American, British Airways, Southwest Airlines, Volaris, West Jet and Condor were all affected by the incident.

The disturbance caused a handful of delays for outbound flights.