Chelmsford police are hunting a man who held a knife to a woman's throat and robbed her on Saturday.
In a previous attack in the town, the knifeman tied up his victim before escaping with cash.
Ds Pete Redman, of Chelmsford CID, said: "We fear that he may do the same thing again.
"He is dangerous. There could be other victims who have not come forward. I would appeal to them to do so. We want to catch this man."
He stressed that they would be dealt with in strict confidence and their anonymity protected.
On Saturday, the knifeman again took cash -- about £100. His 39-year-old victim shouted, screamed and bravely fought back, scratching his face.
She said: "I don't want this to happen to anyone else. This is why I am speaking to you. He is very dangerous."
Saturday's attack happened in Vicarage Lane, Great Baddow, around 3.15pm. The previous incident happened in Hill Road, Chelmsford, in November.
On Saturday, the victim opened her front door to a man who produced a large camping knife.
"Apart from the Hill Road attack, I think he has done this before and it hasn't been reported," she said.
"This man has got to be caught. I know he is going to do it again. He wanted money. This is not sexual. I gave him money.
"I kicked him and scratched him on the face and he ran off. If anyone else has been attacked and not reported it, they should tell the police.
"He became menacing and was waving the knife around. I shouted and screamed. People heard me. There are witnesses who saw him."
The knifeman, who wears a baseball hat back to front, is described as being of stocky build and about 5ft 7in tall. He has dark hair and was wearing dark-rimmed glasses which hid part of his face, a red sweat-shirt, a dark blue bomber jacket, blue jeans and training shoes.
Anyone who recognises the description or who saw the attacker running away from Vicarage Lane is asked to contact police on 01245 491212 or Crimestoppers on 0800 555111
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