WASHINGTON, June 22, 2009 -- At least six people were killed and scores injured during rush hour Monday evening when two Metro trains collided, causing cars from one to land atop the the other.

The National Transportation Safety Board and the FBI are on the scene of what is being called the deadliest crash in the Metro's history, according to D.C. officials.

District of Columbia Fire Department spokesman Alan Etter said crews had to cut people out of the mangled train cars in what he described as a "mass casualty event." Rescue workers propped steel ladders up to the upper train cars to help survivors escape.

According to an update posted on the Metro Web site, there are at least "one hundred injuries, many serious, according to preliminary reports." One of the victims is a female Metro employee operating one of the trains.

Homeland Security Department spokeswoman Amy Kudwa told The Associated Press that federal authorities had no indication of any terrorism connection at this point in the investigation

A Metro train car was stalled, waiting for clearance into the Fort Totten station, when a second one plowed into it from behind, according to D.C. fire officials. The accident happened around 4:59 p.m. on the city's red line in northeast Washington between the Takoma Park and Fort Totten stations.

The female train operator who was killed, whose name has not yet been released, was driving the rear train.

A D.C. alert Monday evening said, "Metro reports that two trains collided and one train is on top of the other train." At 8:17 p.m., a National Transportation Safety Board spokesman said he did not know which train was on top -- the stalled train, or the trailing one.

D.C. police chief Cathy Lanier discouraged family members from coming to the collision scene to retrieve or inquire about loved ones, to avoid overwhelming emergency workers.

Lanier advised family members to call (202) 727-9099 for more information. However, calls by ABC News to that number were met with a busy signal. A secondary number issued by local district officials is (202) 737-4404.

Lanier said a reunification site for family members has been set up, and residents can call 3-1-1 for more information.

According to D.C. fire chief Dennis Rubin, 76 people were taken to various hospitals, including Howard University Hospital and Washington Hospital Center. At least 50 were walking wounded, around 14 suffered non-life-threatening injuries, and six suffered critical injuries, he said.

Dr. Johnnie Ford, a Howard University Hospital emergency room physician, told the AP that a 14-year-old girl suffered two broken legs in the accident.

Metro general manager John Catoe said the cause of the collision was undetermined. At an 8:30 p.m. press conference, Catoe said Metro was still in "investigation mode."

The NTSB has dispatched a go-team to investigate the collision. Railroad Investigator Ed Dobranetski is leading the nine-person team, which includes two specialists from the NTSB Office of Transportation Disaster Assistance, Debbie Hersman and Terry Williams.

"The NTSB is looking at operations, we're looking at maintenance, we're looking at the tracks," Hersman said. "We'll be looking at performance of the equipment as well as survival factors."

Passenger Jodie Wickett, a nurse, told CNN she was seated on one train, sending text messages on her phone, when she felt the impact. She said she sent a message to someone 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," she said.

Wickett said she stayed at the scene and tried to help. She said "people are just in very bad shape."

"The people that were hurt, the ones that could speak, were calling back as we called out to them," she said. "Lots of people were upset and crying, but there were no screams."

One man said he was riding a bicycle across a bridge over the Metro tracks when the sound of the collision got his attention.

"I didn't see any panic," Barry Student said. "The whole situation was so surreal."

Past accidents on Washington's Metro have included a January 2007 train derailment near the Mt. Vernon Square Station on the green line, in which 23 passengers were transported to local hospitals for treatment and released; a November 2006 accident where a train struck and killed workers in Alexandria, Va.; and a May 2006 accident where a train struck and killed a worker near the Dupont Circle Station.

The Associated Press contributed to this report.