By 6:30 p.m., authorities had tracked down and arrested Rodriguez, suspected of raping and strangling three women since November. He was being held on outstanding bench warrants.

Detectives planned Monday night to take another DNA sample, which they expected to conclusively link him to the killings that have gripped the city.

“To the people in the Kensington area, they can rest easy,” said Homicide Capt. James Clark. “We believe we have this killer off the street.”

Rodriguez, a convicted felon, was required to submit his DNA when he was released from jail this summer. It had been waiting to be uploaded into the state police database since Oct. 25, because of a backlog that averages about eight weeks.

Philadelphia police submitted DNA taken from the crime scenes on Nov. 23. Had his DNA been in the system then, he might have been identified about three weeks before the final victim was killed.

Clark and Police Commissioner Charles H. Ramsey declined to comment Monday evening on the backlog, a long-standing problem that plagues police departments across the nation.

“Pennsylvania state police recognize the need to devote additional resources,” Maj. Kenneth F. Hill, director of the state Bureau of Forensic Services, said in a statement, “and used all available means to address the issue.”

Rodriguez was being held for violating probation. Further charges await the results of the second DNA test. Lab technicians at the department’s Forensic Science Bureau were waiting Monday night to begin the testing.

Mayor Nutter on Monday night praised the work of investigators and said he was optimistic that Rodriguez was the strangler.

“Somewhere in the course of the next 24 hours, I’ll also have that greater sense of comfort, knowing that we absolutely, positively, 100 percent have the person off the streets,” he said. “We never stopped looking for him. There was never a break in the action.”

There was little information available Monday night on Rodriguez, whose family lives in North Philadelphia, close to the western edge of Kensington. Several of his relatives were taken to the Homicide Unit to be interviewed Monday night, and a man at the family home who said he was Rodriguez’s brother declined to comment.

Amanda Rios, 22, grew up in the neighborhood and said she had known Rodriguez since he was a little boy.

“That is so crazy,” she said when she learned that her neighbor was a suspected serial killer. “I walk these streets every single day.”