Police are hunting a thief who threatened a teenage girl with a knife and stole her handbag.
The incident happened on Friday at about 2300 GMT near St Giles Church, in St Giles, Oxford.
The 17-year-old was with a friend when they were approached by a man who threatened them a knife before taking one of their bags and running off.
Detectives want to trace a man who ran after the attacker and then returned to make sure the girls were safe.
Grateful thanks
A Thames Valley Police spokesman said: "This was a serious offence that has left the two girls very shaken.
"We are keen to trace any witnesses to the incident particularly the man who helped the girls as he is an essential witness.
"He is around 50-years-old and had short grey hair.
"We not only want him to help in our inquiries but to thank him for his efforts on Friday night."