The number of officers investigating three rapes and two abduction attempts in Northampton has almost doubled.
Northants Police poster are warning woman to be vigilant
About 50 officers are now working on the inquiry with CCTV footage of the town currently being examined.
The main suspects in the inquiry are thought to be three or more black men with South African accents.
Detectives said they had received 1,600 calls from members of the public offering information on the offenders.
Detective Chief Inspector John Jones said: "I am confident that the net is closing in on those responsible. We are following up some significant lines of inquiry."
High profile police patrols are also in place at the weekend to offer reassurance after the attacks on lone women in the town over the past fortnight.
Uniformed and plain-clothed officers are carrying out foot patrols in the areas of the attacks.
Mr Jones said: "Patrols will be more targeted in the key areas to ensure everyone's safety."
South African link
Police are trying to find the cars used by the men to cruise the centre of Northampton looking for victims - a blue 7 Series BMW and a light-coloured Fiat Punto.
The latest victim, a 30-year-old woman, was found with head injuries in Exeter Place, Northampton, early on Monday morning. She had been hit on the head before being raped.
On 1 August attempts were made to abduct a 15-year-old near Sol Central in the early hours of the morning.
Less than an hour later an 18-year-old walking in Horsemarket was forced into a car, driven to Duston and raped.
After police released details of these attacks an 18-year-old woman reported being raped the day before.
She was approached in Horseshoe Street by a man who put his arm around her and walked her across Marefair into a side road where he raped her on a grassy area.
On 3 August a 25-year-old woman was walking across Cheyne Walk when a BMW stopped near her. A man got out and followed her as she ran into Northampton Hospital car park.
He grabbed her and said he wanted to give her a lift home, but the woman said she was due to meet friends and ran into the hospital.