St. Louis authorities are investigating why a small amphibious plane landed in the Mississippi River near the Gateway Arch, setting off a brief panic.

The plane landed shortly after 3 p.m. Wednesday near the grounds of the Arch, then took off again. Drivers on the nearby Stan Musial Veterans Memorial Bridge called authorities.

The fire department first tweeted that it was responding to a report of an aircraft in the river. A Twitter update from the department 10 minutes later said it appeared that an amphibious aircraft intentionally landed.

Authorities still aren't sure why the plane landed in the river. It wasn't immediately clear whether any laws were broken.