Police were searching for a motive for a pipe-bomb explosion outside a theater owned by a former star of TV's "The Sopranos." The actor said he was at a loss to explain the blast. New York Police Department spokesman Paul Browne said there had been "no known threats to the location or its owners" before the explosion at 1 a.m. Tuesday outside Studio Dante, a Manhattan theater owned by actor Michael Imperioli. He played Christopher Moltisanti, Tony Soprano's nephew, on the popular HBO television show. Imperioli told the Daily News he was "completely baffled" by the blast, saying he was unaware of any conflicts with the building's tenants or neighbors. "This whole day felt like a hallucination. It's surreal," Imperioli said.