Detectives have revealed the chilling tactics used by a man dubbed the Thursday Night Rapist.

There have been 13 indecent assaults and one rape during a four-year period in the Chichester and Bognor areas.

Police involved in Operation Bobcat yesterday revealed more information about the hunt and how the search is being extended.

There is now a £20,000 reward for information leading directly to his arrest and conviction.

More than 200 men have so far had samples of DNA taken from the inside of their cheek to be stored in a refrigerator in a secure room at Bognor police station.

The hunt began after the rape of a woman in the Avisford Park Hotel, Walberton, near Arundel, in May.

A DNA sample from the crime was found to match DNA taken from an attack on a woman in the Chichester area two years earlier.

It confirmed police suspicions that one man has been behind a series of attacks on women in the two towns.

Six women have been attacked indoors and another eight indecently assaulted in secluded areas such as footpaths and subways.

Some victims have been tied up so roughly during the attacks they developed breathing problems.

The mass DNA testing of potential suspects follows a vast number of calls to the incident room and an appeal for help on the BBC's Crimewatch.

Police said some victims may have been stalked to find out if they lived alone and whether they kept doors or windows open.

In some cases victims had earlier received nuisance telephone calls and another received a text message via her mobile phone, which intimated she had been watched.

Detective Chief Inspector Steve Scott said: "We do not know if this was a sick joke by a friend who has not come forward and admitted it but it could also show a degree of premeditation in these attacks."

It is also believed the Thursday Night Rapist has been responsible for peeping tom incidents on the Somerstown area of Chichester as he lined up another victim.

Detectives are now helping some victims complete in-depth questionnaires about their lifestyle, hobbies and social and work life.

It is hoped the questionnaires will establish a pattern between the victims, which made them targets for the Thursday Night Rapist and allowed him access to their phone numbers.

Several victims have been students so police officers have been touring campuses offering safety advice to freshers.

Mr Scott says the DNA sampling is "intelligence led" involving men whose description and habits match those of the man he has been hunting for four months.

None of the samples are compared against other crimes and all, except the one that leads police to their man, will be destroyed.

DNA remains the best evidence that will put the man behind bars and police have also revealed how Interpol has been approached to check the Walberton sample with a European database of DNA taken from sex crimes committed in Europe.

Mr Scott said police were not supposing the man was, for example, a lorry driver who may have attacked women during overseas trips.

But he added: "It shows that we are going down every avenue we possibly can."

Operation Bobcat has led to other arrests. During the course of the hunt police have charged a man with four unrelated indecent assaults, another man has been cautioned and a third will be arrested soon also for indecency offences.

Mr Scott said: "This man will be caught. We have his DNA from two attacks and with the continued support of the public still providing us with information we will be knocking on the right door."

Anyone with information should contact 0845 6070999 ext 25751.