SACRAMENTO, Calif. >> Investigators on Sunday were trying to determine what motivated the driver of a sport utility vehicle to ignore a downed crossing arm and flashing lights and pull the vehicle into the path of an oncoming commuter train in Sacramento.

Three died after the Saturday afternoon collision south of downtown, including a 21-month-old boy.

One of the four people inside the Nissan Pathfinder remained in the hospital Sunday at the University of California, Davis Medical Center in Sacramento, where she was being treated for serious injuries.

Authorities also were trying to sort out the relationships of those involved and were not releasing their identifications.

In addition to the toddler, the dead included a 25-year-old woman and a 62-year-old man, who was ejected from the Pathfinder when it was struck by the southbound light rail train traveling at 55 mph shortly after 4 p.m. The impact pushed the SUV about 30 yards down the track and flipped it.

Officer Laura Peck, a spokeswoman for the Sacramento Police Department, said the woman taken to the hospital was the man’s wife.

Investigators and officials with the Sacramento Regional Transit District said video from cameras mounted on the intersection showed the SUV drive around the crossing arms just before impact. That video and other pictures captured by a camera mounted on the train are part of the investigation and were not being released publicly, Peck said.

Witness accounts appear to support the video evidence that the crossing arms were down and warning lights were flashing when the SUV tried to get across the tracks.

Davis resident Ravin Pratab, 42, was in a car that was waiting to cross the tracks when he said he heard a loud bang and then "saw a light-rail train heading south with a big truck smashed on it."

Authorities said six of the roughly 50 passengers on the light rail train were taken to local hospitals but had only minor injuries.

One question investigators are trying to answer is the length of time the crossing arms were down. The light rail train passed through the intersection after two Union Pacific freight trains, going in opposite directions and using different tracks, had passed by.

Neither Peck nor a spokeswoman for the regional district said they knew the length of the interval between the time the freight trains cleared the intersection and the commuter line came through. The light rail system has its own dedicated tracks.

Drivers in Sacramento often can wait up to 10 minutes for a freight train to pass, then might have to wait several minutes more because of an approaching light rail train. The extended wait times can be a source of irritation — and missed appointments — in California’s capital.

Alane Masui, a spokeswoman for the regional transit district, said she could not verify how long the crossing arms were down because that detail was part of the ongoing investigation.

Sacramento’s light rail system carries an average of 50,000 passengers a day. On weekdays, it’s packed with those commuting between the suburbs and state government jobs downtown.

Masui could not immediately say whether Saturday’s collision was the deadliest in the system’s history or how many collisions between light rail trains and vehicles had occurred in the past.