"They're as good as anyone can be," Waters said. He said that the girl was receiving trauma counseling and that police were tracking down the suspects, who are believed to be 13 to 15.
Police were called to the center around 7:30 p.m. Monday after a staff member saw boys running from a stairwell. The staff member found the girl, distraught, in an upper stairwell.
The girl reported that she had been approached by four boys who started talking to her, then cornered her in the stairwell and assaulted her, police said.
"Those people had no fear or respect for anything," Waters said. "It's a rec center. What else do children have that's theirs?"
No arrests have been made, but investigators are speaking to people in the neighborhood, said Capt. John Darby, head of the Special Victims Unit.
"The investigation is proceeding," Darby said. "I'm confident that we will resolve this."
Parks and Recreation Director Susan Slawson urged anyone with information to come forward.
"It's outrageous," she said. "It's heartbreaking."
The department is working to get security cameras in the city's larger recreation centers. Slawson said consultants walked through Francis Myers two weeks ago to decide where they would be placed.
Parents picking up their children from the center's day camp Tuesday afternoon were worried about how the assault would affect the camp and other programs.
Lisette Burgos, whose 3-year-old son attends the camp from 8:30 a.m. to 3 p.m., had nothing but praise for the camp teachers. "It's terrible that that happened," she said. "It scares me, but I don't want it to affect this. This is a good program."
The facility offers numerous programs to keep children occupied and offer a refuge from summer heat - swimming and sprinklers, health fairs, karate, a game room, a gym, and a playground.
On Monday night, the gym was being used for line dancing and basketball. Slawson said the center was "blooming with activity [Monday] night. It was filled with people."
Several parents said that the evening hours at the center are less structured and less supervised than the day camp.
"They need more grown-ups monitoring" after the day teachers go home, Burgos said. "How was this able to happen? At a city rec center?"
Slawson said staffing was not a problem at the Myers center, which is among the department's largest facilities and hired 30 new staffers to handle summer traffic.
Kesha Morton, who brings her 4-year-old daughter to camp and sometimes volunteers at the facility, also drew a strong distinction between the day camp and the more open evening activities.
Despite Monday's attack, "I would go crazy if this center wasn't here," Morton said as her daughter carried a cup of dirt with a green lettuce sprout, part of a gardening project.
Contact Jessica Parks at 215-854-2771 or firstname.lastname@example.org.
Inquirer staff writer Allison Steele contributed to this article.