NEW YORK - American billionaire financier Thomas H. Lee, considered a pioneer of private equity investment and leveraged buyouts, died at the age of 78, his family said in a statement on Thursday, without noting the cause of his death.

The New York Post, citing unidentified police sources, reported Lee was discovered dead on Thursday morning at his Fifth Avenue Manhattan office, headquarters of his investment firm, from a self-inflicted gunshot wound.

According to the Post, he was found after police responded to an emergency-911 call around 11:10 a.m. (1610 GMT)

Reuters could not immediately confirm the cause of death. The New York Police Department said emergency medical service personnel responding to a 911 call on Fifth Avenue at about that time found a "male who was pronounced dead at the scene."

Police gave no further details, and said the city medical examiner's office would determine the cause and manner of death.

The coroner's office could not immediately be reached for comment.

"The family is extremely saddened by Tom's death," Lee's family said in a statement. "Our hearts are broken. We ask that our privacy be respected and that we be allowed to grieve."

Lee was the founder and chairman of Lee Equity, which he formed in 2006, and previously served as chairman and CEO of Thomas H. Lee Partners, which he founded in 1974, according to a biographical statement released by his family.

Over the past 46 years, Lee was responsible for investing more than $15 billion of capital in hundreds of transactions, including the acquisition and subsequent sales of such brand names as Snapple Beverages and Warner Music.

He was also known as a philanthropist and trustee who served on the boards of many organizations, including the Lincoln Center, the Museum of Modern Art, Brandeis University, Harvard University and the Museum of Jewish Heritage.

(Reporting Greg Roumeliotis in New York; Additional reporting by Akriti Sharma in Bengaluru and Steve Gorman in Los Angeles; Editing by Sandra Maler and Leslie Adler)