WASHINGTON -- At least six people were killed during D.C. rush hour, Monday, after a Metro train collided with another in what's being called the worst accident in the rail's 33-year history. 

The first train was waiting for the go-ahead on the tracks between the Takoma and Fort Totten stations on the Red Line, when a second train hit it from behind.  Both trains were hauling six cars and were capable of holding 1,200 passengers each.

The impact was so severe that many of the cars were either smashed together or jumbled on top of each other. 

It's unclear how many were on board or why the second train did not yield to the first.  The National Transportation Safety Board is now heading an investigation.

Monday's collision took place above ground around 5 p.m. ET, according to officials.  By 9 p.m., Fire Chief Dennis Rubin said workers had treated at least 76 people taken off the trains, which rescue crews had to cut apart in order to get inside.

D.C. Fire and EMS Department spokesman Alan Etter called the crash a "mass casualty event."  The female operator of the train was among those who died.  Her name has not yet been released.

Passenger and nurse Jodie Wickett said she was sending a message on her phone when she felt the impact, and texted that it felt like the train had hit a "bump."

"From that point on, it happened so fast, I flew out of the seat and hit my head," Wickett recalled on CNN.

She stayed at the scene to try and help those who'd been injured.

"People are just in very bad shape," she said.

All trains were halted between the Brookland and Fort Totten stations, while crews worked to get people off the trains.

Shuttle service was also established, and Metro spokeswoman Cathy Asato advised riders to avoid the Red Line altogether Monday evening.

The cause of the collision has not been determined, but Department of Homeland Security spokeswoman Amy Kudwa confirmed that there's no indication of a connection to terrorism.

Sources: CBN News, Associated Press, CNN