The fire that gutted the historic St. Leo’s Catholic Church in Tacony over the weekend was intentionally set, federal investigators said Thursday in announcing a $20,000 reward for information leading to an arrest.

But despite classifying the blaze as an arson, the Philadelphia Fire Marshal’s Office and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives remained tight-lipped about how they believe the two-alarm conflagration started.

The church at the corner of Keystone Street and Unruh Avenue ignited roughly around 5 p.m. Sunday, sending black smoke billowing across the neighborhood. It took more than 100 firefighters more than two hours to bring the blaze under control.

By then, its roof had collapsed, leaving only the building’s charred stone outer walls standing. A wrecking ball knocked down what remained on Monday, as dozens of neighbors gathered to watch the final moments of a cherished building where family members had been baptized, married, and memorialized for generations.

The fire came just weeks after the church had been sold to a private real estate company. Representatives for the new owner — Paul Street Real Estate LLC — could not be reached for comment Thursday.

Designed by architect Frank R. Watson and opened in 1894, St. Leo’s was named after Pope Leo I and built to accommodate the neighborhood’s growing Irish population, including many who worked for the Disston Saw Works. A classic example of 19th-century Gothic Revival architecture, it was placed on the Philadelphia Register of Historic Places in 2019.

Its striking bell tower — visible from I-95 — and its elaborate stained-glass windows bearing the names of some of the neighborhood’s founding families made the church a Northeast Philadelphia landmark.

“For a simple church in a simple rowhouse working-class neighborhood, they had the best things in there,” said Celeste Morello, who prepared the historical research for the church’s addition to the registry of historic places. “They had the artwork, the tiles, and all the things that churches in more well-off neighborhoods had.”

The Archdiocese of Philadelphia closed the parish in 2013 as part of a cost-cutting merger with neighboring Our Lady of Consolation Parish, but the building was kept open for occasional services until it was permanently shuttered in 2019 to prepare the building for sale.

That transaction closed in April, an archdiocesan spokesperson said Thursday. All sacred liturgical items had been removed.

The fire remains under investigation by the Philadelphia Fire Marshal’s Office, ATF, and Philadelphia police.

The $20,000 reward is being offered by ATF and the Citizens’ Crime Commission.

Investigators urged anyone with information about the fire to call 1-888-ATF-TIPS, email ATFTips@atf.gov, or submit a tip through the website www.atf.gov/contact/atftips or the mobile app ReportIt.