Investigators were trying to determine the cause of the 4-alarm fire that tore through a historic Flatiron church, and police said the arson squad was assisting Monday.

The fire gutted the Gothic-style Serbian Orthodox Cathedral of St. Sava on 25th Street following the Orthodox Easter services there.

On Monday, the smell of smoke still hung in the air. The roof of the grand church appeared a skeleton of its former self and debris littered the ground.

The blaze broke out just before 6:50 p.m. Sunday and took nearly three hours to be brought under control, according to the FDNY. Five people suffered minor injuries, including four firefighters and one civilian who had minor injuries but refused medical treatment.

Parishioners stopped by to survey the damage, several who had been attending services at there for decades.

Zoran Milojevic, 44, said the church served as a social meeting place for many Serbians throughout the city. Milojevic, a stock broker who lives in Somers in Westchester County, said people would even use the church’s network to look for jobs.

“From cultural activities, to [an] artists place, to seeing a friend you haven’t seen in a while, to actually trying to find a job,” he said, adding the church was the hub. “It was also a networking place — everything for the people that were here second, third, fifth generation to the newcomers.”

Milojevic has attended the church for about 20 years and said his parents renewed their vows there as well.

Upper West Side resident Alexander Doncov, 33, attended services with his family Sunday morning. Doncov, who was married in the church about six years ago, found out what happened on his way home.

“It was really a cornerstone of Serbian immigrants who come here over the years. It was sort of a center of this society and it’s a shame to see what happened,” he said. “Thank God it didn’t happen during the service.”