Philadelphia 76ers owner Joshua Harris has pledged to donate $3.5 million to the Philadelphia Police Athletic League, the largest gift ever recorded by the nonprofit youth group, The Inquirer learned Wednesday.

The New York-based investment tycoon planned to announce his philanthropy at an 11 a.m. news conference Thursday at a PAL center in the Harrowgate section.

The donation is being made through a family foundation affiliated with Harris, a graduate of the University of Pennsylvania's Wharton School who cofounded Apollo Global Management L.L.C.

PAL board member Gene Castellano said Wednesday afternoon that officials with the nonprofit would not comment on the size of the planned gift or the donor's identity. Two sources confirmed the news.

A spokesman for Harris, Charles Zehren, declined to comment. Messages left with PAL spokesman Chris Lyons were not returned. A top Sixers executive also was not reachable about the planned gift.

The 68-year-old group is expected to announce the gift at the center at 851 E. Tioga St. In addition to Harris, speakers are expected to include Police Commissioner Charles H. Ramsey; City Councilwoman Maria Quiñones Sánchez; the PAL's commanding officer, Lt. William Eddis; and PAL board chair Jeffrey Hendrey.

Harris became a billionaire by investing heavily in distressed properties, including the stranded assets of European banks, such as corporations, real estate, and consumer loans.

Since becoming the Sixers owner, he has kept a low profile, speaking to reporters only on rare occasions, as those entrusted with the operations of his NBA team have overseen several seasons of extreme losses, all in the hope of building a powerhouse roster through the annual player draft.

Before launching Apollo, he worked at the former Philadelphia firm Drexel Burnham Lambert.

Harris' gift is considerable for a group whose revenue in 2013 was $2.2 million, according to PAL tax documents.

Eighteen PAL centers across Philadelphia serve thousands of boys and girls with sports, educational, and cultural programs. Each is staffed by paid members of the Police Department, whose labor is donated by the city as an in-kind gift. In 2013, the value of that donated labor totaled $3.86 million, according to tax documents.

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Inquirer staff writer Marc Narducci contributed to this article.