The death toll had been revised down for several hours on Tuesday after Mr Fenty said just seven people had been killed.
Passenger Maya Maroto, who was on board the moving train, said it had been going at "full speed".
"I didn't hear any braking. Everything was just going normally. Then there was a very loud impact. We all fell out of our seats.
"Then the train filled up with smoke. I was coughing," she told the Associated Press.
Theroza Doshi told Reuters news agency: "There was no slowing down of the train, just a jerk. There was no attempt at braking.
"We just slammed into whatever we slammed into."
Passenger Abra Jeffers, 25, told the AFP news agency: "I was on the train that got hit. I thought it was an explosion. I thought it was like the train bombings in London. There was smoke and dust everywhere."
On Tuesday morning, Mr Fenty said two of the patients with critical injuries remained in hospital.
The driver of the moving train was among the dead, and was named as Jeanice McMillan, 42, by the Associated Press (AP) and the Washington Post.
Washington fire chief Dennis Rubin said a large crane had been used to separate mangled pieces of wreckage so that rescuers could search for injured or dead.
He said parts of the lower carriage were 70 to 80% compressed, and that rescuers did not know if there were still more bodies to be found.
A thorough search of the sidings and surrounding woodland had been made, he said.
Survivors of the crash describe the accident
Investigators have been trying to find recorders that would have details of the train's speed at the time of the crash and other information which could explain how the accident happened.
The accident happened at the peak of rush hour, at 1700 local time (2200 BST) on a busy commuter line.
But the trains involved were heading towards the centre of Washington rather than to the city's outlying areas.
This meant they were likely to have had fewer people on them, Ms Hersman as said.
The accident is the 33-year-old Metro network's first crash with any passenger fatalities since 1982 when three people were killed in a derailment.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.