Detectives from the Major Crime Unit have never closed the investigation into two sexual assaults that happened in Harpenden over 20 years ago - and now suspect a third attempted abduction was committed by the same man.

Between 6pm and 6.30pm on Sunday 7 April 2002, a 16-year-old girl was walking along Piggottshill Lane when a man grabbed her. She was dragged into a nearby field, sexually assaulted and forced her to perform sexual acts.

Two months later, between 6.45pm and 7pm, on Wednesday 19 June 2002, a 17-year-old girl was walking along Wheathampstead Road, when a man forced her into a small red car. He then drove down Piggottshill Lane and took her to a wood where he forced her to perform sexual acts and sexually assaulted her.

The third possibly linked offence happened in 2004 - just after 6pm on Wednesday March 17 and involved a 12-year-old girl walking in the High Firs Crescent and Greenway area of Harpenden. As she walked down Green Way, away from the Wheathampstead Road, she was grabbed from behind and dragged into a public footpath, known locally as the Spinney. The man, who was carrying a knife, threatened and restrained the girl, before a woman intervened and he ran off.

Crimestoppers, which is independent from the Police, is supporting these investigations by offering a reward of up to £10,000 for anonymous information it exclusively receives that leads to the conviction of the man or men responsible.

Information passed directly to police will not qualify for a reward. Only information given exclusively to the charity Crimestoppers via the 0800 555 111 number or the simple and secure anonymous online form at www.crimestoppers-uk.org will qualify.