Dr. Henry Bello walked off his job with New York City's Human Resources Administration in April, saying he had "personal problems," according to city spokeswoman Jaclyn Rothenberg.
Rothenberg said Bello was a caseworker for the city's HIV/ AIDS Service Administration. He stopped reporting to his job on April 11 and was officially terminated on June 21.
"He was let go after repeated attempts to reach him," Rothenberg told CNN on Sunday.
The six people wounded on Friday remained hospitalized through the weekend. Four patients were being treated at Bronx-Lebanon, with one expected to be released Monday, according to the hospital. The other two were at Mount Sinai Hospital, where one of them underwent surgery on Monday.
It may never be known what led Bello, 45, to return to his former place of employment -- Bronx-Lebanon Hospital -- with a rifle hidden under his lab coat, go on a shooting spree and kill himself afterward. But he had a troubled history.
In 2004, Bello pleaded guilty to a misdemeanor unlawful imprisonment charge, according to Caitlyn Fowles of the Manhattan District Attorney's office. He was sentenced to community service. Fowles was unable to provide further information about Bello's case.
Rothenberg said a background check run on Bello before his September 2016 hire by the city did not reveal his 2004 arrest because the charge he pleaded guilty to was not a disqualifying one.
Bello also had financial troubles. He sought unemployment benefits in 2014 and complained about difficulties in getting a US medical license because he went to medical school overseas, according to an attorney who helped him seek benefits.
According to a city official who works with New York's homeless shelters, Bello had been in shelters at least twice since 2003, and had most recently left a shelter in March after securing government assistance for permanent housing.
Bello worked at Bronx-Lebanon Hospital for only six months, from August 2014 to February 2015. Hospital spokesman Errol Schneer told reporters on Friday Bello resigned before he was to be fired.
Bronx-Lebanon has not responded to requests from CNN regarding any background check run on Bello before his employment.
Bello practiced medicine in New York State under a so-called limited permit issued to him in July 2014, according to the New York State Department of Education.
The department told CNN that the permit enabled Bello, a graduate of Ross University School of Medicine on the Caribbean island of Dominica, to practice medicine at Bronx-Lebanon Hospital despite his foreign medical degree.
Ross University did not respond to requests for comment over the weekend, nor did it confirm Bello's attendance or graduation.
The school did, however, issue a tweet
the night after the shooting, stating, "Ross is deeply concerned for and extends heartfelt prayers to all of those affected by the shooting at Bronx-Lebanon Hospital."