A 17-year-old girl was knocked unconscious.
Hertfordshire Constabulary said the teenager was approached by a group of four people in Shenley Lane on Sunday, June 5.
She was struck on the head and left unconscious. But the teen cannot remember any other details about the attack.
Nothing was stolen.
Detective Constable Steve Cook said: “I understand this incident may cause some concern in the community but I would like to reassure people we are doing all we can to establish what happened.
“The incident occurred near a busy stretch of road in broad daylight so it is likely there would have been a number of motorists around at the time.
“I am appealing for anyone who saw what happened or saw anyone acting suspiciously in the area around the times stated to please get in touch.”
Anyone with information is asked to contact police on the non-emergency number 101, quoting crime reference F3/16/909.
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