Fenty said seven people died in the crash at the Metro system track that carries passengers from the US capital into the state of Maryland.
"There was no attempt at braking. We just slammed into whatever we slammed into"
However, The District of Columbia Fire Department website said on Tuesday that three bodies had been found in addition to the six fatalities reported earlier.
Rescue workers used cutting equipment to free people from the wreckage following the incident.
The collision on an above-ground stretch of track caused the front of one train to jackknife into the air and ride up on top of the other stationary train.
John Catoe, the subway system's general manager, said the first train was stopped on the tracks, waiting for another to clear the station ahead, when the trailing train, one of the oldest in the network's fleet, ploughed into it from behind.
Transport authorities said one of those killed was a female train operator.
The accident occurred on the Metro system's red line between Fort Totten and Takoma stations on the northeastern edge of the city near the border with Maryland.
|Officials from the National Transportation Safety Board are investigating the crash [AFP]
Both trains were heading south into the city and so were less likely to have been packed with commuters.
"It just happened. There was no slowing down of the train, just a jerk," passenger Theroza Doshi told Reuters news agency.
"There was no attempt at braking. We just slammed into whatever we slammed into."
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.''
Wickett said she remained at the scene and tried to help.
"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.''