(CNN) -- The bystander's video starts with a chaotic scene: An ambulance runs through an intersection in New York, one of its doors open, lights flashing. It drags what appears to be a person trapped under the vehicle.
Authorities say the person stuck under it was Yadira Arroyo, 44, an emergency medical technician who died in the incident in the Bronx on Thursday night.
Her partner was injured.
'He ran over our ... EMT'
The incident started when a man jumped on the ambulance's bumper on its way to respond to a call, FDNY Commissioner Daniel Nigro said.
A passing motorist alerted the EMTs to the man riding on the back of the ambulance, he said.
When Arroyo and her partner exited the vehicle to investigate, the man jumped inside, according to authorities.
He put the vehicle in reverse and ran over Arroyo, authorities said.
"They got out to see what has happening," Nigro said. "This person in turn went around the other side of the ambulance and got into the driver's side ... they tried to get him out of the ambulance, he put it in reverse he struck the one EMT who was injured, and he ran over our other EMT and she was subsequently killed."
The ambulance then stopped after it crashed into parked cars and a pile of snow, video posted on social media shows.
"It's a sad night for everybody in the department," Nigro said. "This person has no business being in this ambulance."
An off-duty Metropolitan Transportation Authority officer intervened, along with several good samaritans, authorities said. The suspect, a 25-year-old man, is in custody.
"A lot of heroism was on display today amidst a real tragedy," Mayor Bill de Blasio said.
Arroyo was a 14-year veteran of the FDNY, the mayor said. She has five children.
"We lost a good woman," he said.
The second EMT, also a woman, suffered minor injuries.
Arroyo is the eighth member of New York's emergency medical services to die in the line of duty, Nigro said.
Of the eight fatalities, three have been women.