Credit: Via Pete Piringer/Twitter

Updated 8:50 a.m.: About two dozen people were injured and all southbound lanes of I-270 were closed for two hours Monday morning after about 20 vehicles crashed on the highway near Middlebrook Road in Germantown, according to Montgomery County Fire & Rescue Service spokesman Pete Piringer.

Medics have taken four people to the hospital, including two trauma patients, according to Piringer. None of the injuries were considered life-threatening. EMS teams declared the crash a “mass casualty incident.”

 The multivehicle crash occurred at about 5:30 a.m., Piringer wrote on Twitter. One vehicle was a truck carrying slabs of slate that then fell onto the highway.

Southbound I-270 was closed between Middlebrook Road and Montgomery Village Avenue. Two lanes were opened at about 7:30 a.m. All lanes were reopened by 8:50 a.m.

This story was updated at 4:50 p.m. to correct a reference to a truck carrying rocks. The truck did not overturn, and the rocks were not the initial cause of the crash.