Detectives investigating a series of rapes in Northampton have issued a reward for information leading to the arrest of the sex attackers.
Northants Police poster are warning woman to be vigilant
Over the last two weeks there have been three rapes and two attempted abductions which Northamptonshire Police believe could be linked.
The attacks have led police to warn women and girls not to go out alone in the town after dark.
On Thursday, a £1,000 reward for information was announced by police.
About 200 posters have been put up in Northampton town centre urging women and girls to be vigilant and security conscious.
Det Ch Insp John Jones said dozens of people had called the incident room after a media appeal.
He said: "The response from the public has been fantastic with some very useful information, which we are following up with an increased team of officers.
"A very kind member of the public who has asked to remain anonymous has offered a £500 reward and the force has matched that amount to provide a reward totalling a £1,000."
The offenders are described as black, in their 20s, with very dark skin, about 6ft tall and spoke with South African accents.
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.
Hospital car park
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.