Police in the Indian capital suspected a psycopathic serial killer may be behind the headless bodies dumped outside a high-security prison in the city, media reports said Saturday. A headless body, stuffed in a sack, was found outside the high-security Tihar jail on Friday, dpa quoted the Times of India newspaper as reporting. This is the fourth body found outside the jail since December 29, 2005. A letter found near the body challenges the police to catch the killer, the newspaper reported quoting police sources. This is the second time such a letter has been found near one of the bodies. The police said there was a definite pattern emerging from the four incidents, but there were very few clues. They have not been able to identify any of the victims, whose limbs and some other body parts were missing. The police suspect the bodies are of vagabonds. All of them are male aged 25-35. All the bodies were found early in the morning. Jail officials are working with the Delhi Police to increase security outside the prison. The letter-writer claims he was wrongly framed in a murder case for which he had to serve a prison term. "I will continue killing people and dumping their bodies here. Catch me if you can," the letter written in Hindi says, according to the police. The forensic department has been asked to inspect the letter. An earlier letter found near a headless body, left in a similar gunny sack outside the jail on October 28, 2006, did not provide any leads. Investigators have approached prisons across the country seeking information of any person who was wrongly convicted for murder and released in 2005.