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PHILADELPHIA (CBS) — A man wanted in connection to several sex assaults and robberies in West Philadelphia has turned himself into police.

Philadelphia police say 51-year-old Robert Johnson was calm when he surrendered to Southwest Detectives around 7:30 a.m. Thursday. Police say the convicted sex offender was released from prison last year.

Detectives served a search warrant at Johnson’s home on the 4100 block of Leidy Street overnight, where police say a weapon related to the attacks was recovered. Johnson was not home at the time of the search.

Police say Johnson, who they call a “power rapist,” is believed to have raped three women last week, and sexually assaulted a fourth person last month.

“Well, he struck three times in two days, three sexual assaults we know of,” said Capt. Mark Burgmann of the Special Victims Unit. “So yeah, there’s always that fear someone is going to strike again. Honestly, I did believe he was going to strike again.”

Burgmann says the suspect attacked a woman around 5:45 a.m. last Tuesday on Concourse Drive in Fairmount Park, moments after she got off a bus. She was able to escape from the man and call authorities.

Two days later, detectives say the same man forced a woman into a car at a gas station on 48th and Girard Avenue. Detectives say it happened just after 4 a.m. The woman was taken to Fairmount Park and raped with another woman, according to authorities.

Walker says he believes the suspected rapist is behind two other robberies in June. On June 21, a woman was forced into a car in West Philadelphia and driven to two ATMs on 52nd and Jefferson and 36th and Chestnut. On June 30, an Uber driver and two passengers were robbed after leaving The Mann Center.

Police say Johnson is also believed to be connected to a fourth sexual assault on June 30.

“Honestly, I did believe he was going to strike again,” said Burgmann.

If you have any information on these incidents call Philadelphia Police at 215-686-TIPS. You can remain anonymous.