He did not say what that vehicle was, but San Francisco Fire Chief Joanne Hayes-White said officials believe the girl was struck by a specialized fire vehicle.
"It's very difficult and devastating news for all of us," Hayes-White said.
She said she has reached out to the girl's family through the consulate and wants to meet them.
Investigators believe the teenager was on the ground and not standing up when she was struck, Hayes-White said.
San Francisco police and the National Transportation Safety Board are investigating what led to the girl being hit, but the chief said she could not comment on the probe.
She described the scene firefighters faced as "volatile" and "dangerous" with flames and leaking fuel.
Foucrault declined to go into detail on how investigators determined the teenager was alive before she was struck, but said they could tell from internal hemorrhaging.
Authorities confirmed last week that Yuan was hit by a vehicle racing to extinguish flames that broke out on the Boeing 777.
Police said she was on the ground and covered in fire-retardant foam that rescuers had sprayed on the wreckage.
Yuan and her middle school classmate, 16-year-old Wang Linjia, died on July 6 at San Francisco International Airport. The other victim, 15-year-old Liu Yipeng, died at a hospital July 12. Dozens of others were injured.
Yuan and Linjia were students at Jiangshan Middle School in Zhejiang, an affluent coastal province in eastern China, Chinese state media has reported.
They were part of a group of students and teachers from the school who were heading to summer camp in Southern California. Yuan and Linjia were seated at the back of the plane, federal investigators have said.
Meanwhile, the probe into the crash itself continues. Investigators have said the plane came in too low and too slow, clipping its landing gear and then its tail on a rocky seawall just short of the runway.