Boston Police Commissioner William Gross said on Wednesday detectives had few details about what happened to the three victims, whom he said were found unconscious on the pavement and pronounced "non-viable" at a local hospital.
Asked during a solemn news conference whether the three may have jumped to their deaths, Gross answered, "I have no way of knowing that, and we definitely won't jump to conclusions or try to speculate."
But aerial footage of the scene aired by local television stations showed police investigators huddled on the rooftop around a black sport utility vehicle left parked with three of its doors open beside the wall enclosing the top level of the structure. The area around the SUV was cordoned off with yellow crime-scene tape.
The Renaissance Park Garage, where the deaths occurred, lies adjacent to a transit station near the Northeastern University campus.
Gross said the cause and manner of death would be determined by the medical examiner and the case was being handled as a "death investigation," headed by Suffolk County District Attorney Rachael Rollins, who also spoke to reporters.
She and Gross both said the tragedy seemed all the more grim for having involved two small children on Christmas Day.
"As a mother, it was incredibly hard, this scene in particular, where there were two young children who lost their lives today," Rollins said.
Authorities appealed to anyone who may have witnessed what transpired or have any other useful information to contact investigators through a tip line.
The identities, ages and relationships of the three victims had yet to be ascertained, according to Gross, but he said the ages of two children were believed to be "5 and under."
Rollins said the victims' vehicle was found at the parking structure, with one child seat facing forward and the other backward."We're going to do everything we can" to solve the case, she said.