City officials confirmed Wednesday that Christopher Husbands — who police allege is in a gang — was employed as a part-time recreation worker from last November until May 18.
Husbands was out on bail on a sexual assault charge while he held the job.
City manager Joe Pennachetti called the discovery "deeply troubling" and said it has led the parks department to impose stricter hiring conditions.
From now on, he said, no one will be hired by the department without first handing over a clean background check.
"I know there's various legalities involved, but at this stage, from my perspective, we don't hire someone — especially when they're working with children — that has a criminal background," he said.
Mayor Rob Ford said he thinks all city employees should undergo a background check, regardless of the department in which they work.
"We have to make people accountable," he said.
Saturday's shooting in a busy food court left one man dead and six others wounded, including a 13-year-old boy who was shot in the head and is now recovering in hospital.
Husbands, 23, was under house arrest at the time of the shootings. He is charged with one count of first-degree murder and six of attempted murder.
He made a brief court appearance Monday and was remanded into custody.
The dead man, 24-year-old Ahmed Hassan, belonged to the same gang as the accused, police said. So did an associate who was badly wounded in the gunfire, they said.
Police have refused to discuss a motive for the violence, but said it appeared to stem from a personal dispute rather than gang matters.
A massive influx of new seasonal staff may have allowed Husbands and other workers to avoid scrutiny during the hiring process, Pennachetti said.
The parks department hires thousands of part-time, temporary workers in each spring and summer, he said, many of them youth looking for work experience.
All those who work with children were already required to go through a police background check, even though it's not mandated by law, he said.
But the process was left to the initiative of employees and it can take up to three months for the results to arrive, the department said.
Pennachetti said he'll be checking to make sure no other employees have slipped through the cracks.
Note to readers: This is a corrected story. An earlier version said Husbands appeared in court Tuesday.