The Charles River is searched Wednesday for a missing swimmer. (Photo: Boston Fire Department via Twitter)

Firefighters went Wednesday from an overnight chemical fire that left 12 of them hospitalized to a rescue in the Charles River by the Boston University Bridge. The 11:20 a.m. call for a water rescue along Memorial Drive had divers swiftly entering the Charles to search for one unaccounted-for swimmer who’d called for help; by around 2:15 p.m. a team had pulled a body from the waters.

If that wasn’t all, at noon there was a third crisis: A construction accident on Main Street, where a fall victim was evaluated for extrication to a medical facility, according to the Cambridge Fire Department’s social media.

At the time of posting, the search of the Charles River was ongoing.

The fire took place at 355 Massachusetts Ave., in The Port near Lafayette Square, a five-story Massachusetts Institute of Technology dorm where smoke began showing at around 12:47 a.m., the department tweeted. Firefighters who entered encountered “a chemical reaction with peroxide and ammonia stored in an apartment” – found when firefighters noticed an unusual odor coming from a vacant room, officials said.

The scene of an overnight fire in The Port near Lafayette Square.

“Units found smoke coming from the floor of an unoccupied unit. When the floor was opened and water was applied to extinguish the fire, it reacted with chemicals that had breached their original containers and created the initial chemical reaction,” according to a report sent by fire department spokesperson. “We are still in the process of investigating why the chemicals were in the apartment and what caused the breach.”

The effects on the dozen firefighters and hazmat technicians, which included chemical burns, were not life-threatening, fire officials said, and they were later released from the hospital. Four residents were displaced from the building. Sarah McDonnell, deputy director of media relations at the MIT News Office, confirmed there were MIT students living in the building, and all were evacuated safely and unharmed.

“We appreciate the help of our local American Red Cross chapter with identifying alternative overnight housing, and our student support professionals are in touch with the students to offer further assistance. We are also grateful for the swift response by the Cambridge Fire Department, which is looking into the source of the incident with assistance from the Cambridge Police Department and MIT Police,” McDonnell said.

In total, some 45 Cambridge firefighters and 25 Boston firefighters worked at the incident. Brookline and Somerville fire companies covered in Cambridge during the incident, the fire department spokesperson said.

This post was updated July 28, 2021, to add that a body had been recovered from the Charles River.