BOSTON >> A woman walking across a Boston drawbridge was crushed to death today after an operator raising the bridge for a boat to pass heard her screams and lowered it, accidentally trapping her between the two plates, investigators said.

The woman was crossing the bridge around noon when a bridge operator, not aware that she was on the bridge, began raising it for the boat in the Chelsea River. The woman grabbed hold of one of the sides of the bridge and the operator immediately lowered it when he heard her scream, but she became trapped in between the plates and suffered massive trauma, police said.

“I couldn’t see her, but I could hear her,” witness Waldina Garcia, 47, told the Boston Globe. “She was screaming and screaming and screaming.”

The woman, who wasn’t identified, was pronounced dead at the scene. Investigators called her death on the Meridian Street Bridge, which connects Chelsea and East Boston, an accident.

The distraught bridge operator was taken to the hospital for evaluation, officials said.

Dot Joyce, a spokeswoman for Mayor Thomas Menino, told the Globe that the bridge operator was placed on unpaid leave pending results of the investigation. She described that as standard procedure.

Investigators told the Globe it wasn’t clear whether the woman began crossing the bridge after warning lights and alarms were activated.

The bridge was closed to traffic during the investigation, but later reopened.