Speaking at Boston Fire Department Headquarters, just below a ceiling display of portraits honoring firefighters who have died in the line of duty, Marshall said the department's Fire Investigation Unit had completed its origin-and-cause review.

"Investigators determined that the fire originated in the rear portion of the building on the third-floor enclosed porch," Marshall said. "The investigation determined that the fire was caused by thermal runaway, involving multiple lithium-ion batteries stored inside of plastic toolboxes."

Firefighters were called to the home at 8:15 p.m. on Tuesday, May 23. About half an hour later, a mayday was declared after Kilduff fell from the third-floor porch. A third-generation firefighter and U.S. Marine Corps veteran assigned to Rescue 2, Kilduff was taken to Boston Medical Center, where he was pronounced dead. His portrait was added to the department's memorial display this summer.

Marshall stressed that while the origin-and-cause investigation is complete, the broader inquiry into Kilduff's line-of-duty death is not. Investigations by the National Institute for Occupational Safety and Health and the Boston Fire Department's Board of Inquiry are ongoing.

"These investigations will examine all of the circumstances surrounding this tragedy," Marshall said. "We will allow these processes to proceed without speculation or premature conclusions."

In the meantime, Marshall and Fire Chief Keith Kelly of the department's Special Operations Command urged residents to handle lithium-ion batteries with care. The rechargeable batteries power everyday items such as phones, cordless vacuums and power tools, as well as larger equipment including electric scooters, drones and grid energy storage systems.

Kelly said batteries should be kept at room temperature, used only on hard, stable surfaces and inspected routinely. He advised watching for swelling, leaks, corrosion or other damage, and never charging a battery that appears compromised. He also recommended buying batteries certified by a nationally recognized testing company such as Underwriters Laboratories and using the charging cable that came with the device.

Kelly pointed to a common hazard: replacing a manufacturer's battery with a cheap online knockoff. "That is a very common cause of thermal runaway," he said.

Kelly, who was a classmate of Kilduff's, grew emotional near the end of the news conference. He said his goal was not to frighten people but to make sure batteries are stored, used and inspected properly.

Mayor Michelle Wu echoed that message. "These batteries power so much of our daily lives, but also carry very serious risks," she said. "We owe it to all of our community members to educate our residents about the dangers and how to prevent fires like this one."

Wu also honored Kilduff's sacrifice. "Bobby Kilduff gave his life in the line of duty, doing what our firefighters do every day — putting themselves in harm's way to protect their neighbors and our community," she said. "He made the ultimate sacrifice in service to our city, and we are continuing to work on several efforts to honor his legacy and support his family."

The findings come as battery safety draws growing attention locally and nationally, with fires linked to lithium-ion cells increasingly reported in homes, e-bike charging and waste facilities. Officials said the department will release further information from the pending investigations when they are complete.