DEARBORN, Mo. - UPDATE: 12/23 1:08 p.m. The incident is over and the suspect is in custody, according to the Missouri Highway Patrol. Both lanes Interstate 29 to reopen.
Workers at Trex Mart, just north of the rest stop, told KMTV's sister station KSHB that they were told to stay in the center of the building in case the gunman starts shooting.
I-29 is being shut down near the Dearborn rest stop after a report of a man pointing a gun out of a semi, according to Missouri Highway Patrol.
The northbound lanes of I-29 are closed at mile marker 24 and the southbound lanes of I-29 are closed at mile marker 30.
Authorities say the man with the gun is situated in the northbound rest area, but was pointing the gun at the southbound lanes.
Troopers are on the scene. They are rerouting northbound traffic at exit 20 and southbound at exit 30.
This is a developing story, check back for updates.