Lisa B. Christensen, a 19-year-old midshipman in the Boston University Naval ROTC, collapsed on the 14th Street Bridge a little more than two miles from the finish of the Marine Corps Marathon yesterday and died at Arlington Hospital.

Hospital officials said the cause of death could not be determined; an autopsy will be performed this week.

Almost immediately after Christensen was stricken, an ambulance was summoned by ham radio operators working as volunteers for the race. She was approaching the 24-mile mark at the Route 110 turnoff from the bridge.

Race officials said she was unconscious with a weak pulse and was initially treated for heat stroke, a common procedure in a race of this length and with temperatures in the 70s. After treating Christensen on the bridge, the responding ambulance from the Wheaton Volunteer Rescue Squad transported her to the hospital, where officials said she died at 2:45 p.m.

Christensen fell among runners who were on schedule to finish in about 4 hours 30 minutes -- a 10-minute-mile pace -- as she had predicted she would on her entry form. Race officials said the ambulance was called at 1:15 and the hospital, one of six local facilities on standby, was notified at 1:28.

"I still don't know what happened. The first story I was told that a runner went down on the bridge and hit her head," said Robert Schultz, the medical coordinator of the race and the medical director at Quantico. "It could be heat exhaustion and she fell and hit her head, which caused a cranial hemorrage. The bottom line is they're all rumors. I don't know if she was spotted by marines, a ham radio volunteer, another runner."

Christensen, a sophomore from Cheshire, Conn., who was majoring in anthropology, came to Washington with 14 fellow ROTC members to run the race.

She ran regularly, three times a week with her training partners, said Capt. Michael Fields, Command Officer of the Navy ROTC at Boston University.

"They were all in good condition," he said. "She listed her only hobbies on the {entry} form as running and swimming. She was in good condition."

In her most recent physical fitness test, given semiannually to ROTC candidates, Christensen scored in the outstanding category, Fields said. Two weeks ago Christensen competed in an interservice track and field meet at the university and ran the quarter-mile and several relays.

The Marine Marathon is popular among runners for its attention to detail. Approximately 300 medical personnel staffed eight field aid stations and the main station at the start/finish area. There were 20 ambulances stationed around the course. Marines run through several trial races, playing out many scenarios. A health emergency is one of them.

"You're aware it can happen and for that purpose you prepare an emergency response," said Capt. Marshall Fields, the race coordinator. "We knew, for instance, that the weather would be warm and possibly hot and we prepared for that by increasing the amount of water on the course and supplies for treating heat related problems and encouraged the runners to drink a lot of water. We prepare for the eventuality and hope it never comes."