YORK, Pa. (AP) — Federal authorities say the cause of a central Pennsylvania fire that preceded a building collapse that killed two firefighters earlier this year will remain undetermined.

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives announced Thursday that it has closed its investigation into the cause and origin of the March 21 fire in a former piano factory in York.

An ATF fire investigator determined that the March 21 fire started somewhere on the first floor of the south wing of the Weaver Piano and Organ Building, ATF Special Agent Charlene Hennessy, who was in York during the investigation, said in a release.

"Due to the extensive damage and lack of evidence, a specific point of origin and ignition source could not be identified, therefore, this fire is classified as 'undetermined,'" the release said.

Ivan Flanscha, 50, and Zachary Anthony, 29, died in a wall collapse the day after the fire as they searched for hot spots. Two other firefighters were seriously injured. Fire Chief David Michaels said the rescue effort to find the four "was quite possibly the most complex rescue in our department's history."

That effort saved the lives of the other two firefighters with "rescue tools, sheer will and brute force," Michaels said Thursday at the department's annual awards ceremony.

Thousands attended a memorial service in March for Flanscha — a 20-year veteran of the department, onetime firefighter of the year and father of five— and Anthony, who joined the department in 2010. Their badges were officially retired Thursday.