As described in the book Dreams Die Hard, Sweeney succumbed to mental illness and became paranoid and delusional, believing that Lowenstein was the central figure in an elaborate plot against him. Sweeney shot and killed Lowenstein in 1980, after which he turned himself in to the police. He was convicted and served eight years in the Mid-Hudson Psychiatric Center, New York's maximum security psychiatric hospital. He was moved to a lower-security psychiatric hospital, and eventually started being released for furloughs for increasing amounts of time. His diagnosis was that of paranoid schizophrenia.
Sweeney was ultimately released from any level of custody in 2000 over the objections of Lowenstein's family (who had previously opposed prosecutorial plans to seek a sentence of death for Sweeney), prosecutors and New York mental health professionals. At the time of his release, he had been off medication for six years and had been regularly given access to the community outside the hospital, being required to spend only one night every two weeks in the hospital. Justice Brenda Soloff of State Supreme Court ruled that he was not a threat and ordered him released.