A Boston University professor was crushed to death by an elevator in her apartment building when it suddenly dropped, trapping her between floors.

French lecturer Carrie O'Connor, 38, was getting onto the elevator with a box on the first floor of the building in Allston about 5.15pm on Monday local time when the lift jolted downward, residents said.

"I heard someone that was bringing in a package out in the hallway, and then I heard an ungodly scream," resident Leanne Scorzoni told a local TV station WCVB.

"Then we ran out into the hallway, and we saw a gentleman who was obviously in distress. He was screaming and hyperventilating, saying, 'She's dead! She's dead!'"

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O'Connor had just moved into the building on Commonwealth Ave a few weeks ago.

A man who witnessed the accident had to be taken to hospital for trauma. A source told The Sun, "he saw things that no one should ever see".

Scorzoni told CBS the man "was helping [O'Connor] with a box into the building, and he was going up the stairs, and he had told her, "Hey just be careful, because … you have to pull the door across and then step in and then press the button."

She said the lift was "old-fashioned".

"If you have something in there, it can trigger a sensor.

"[The witness] believes that whatever [O'Connor] was trying to get in there hit the sensor, and then it started moving."

The elevator car's roof was visible from the lobby after the incident, residents said.

"The car had to have gone at least halfway down," Scorzoni said.

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An autopsy showed that O'Connor died of "traumatic asphyxiation".

The Massachusetts agency responsible for overseeing elevator safety said the lift passed inspection at the 1920 building last year, according to the local NBC station. It was not clear how old the elevator was.