Staff at residential care homes for the elderly are being asked to be extra vigilant after a bed-ridden woman was raped in her room.
The attacker was discovered by a member of staff
A member of staff, at the home in Nottinghamshire, disturbed a naked man attacking the woman in her room at about 0520 BST on Sunday.
The intruder, described as black, slim, aged 20-25 with dreadlocks, ran off.
Det Insp Sean Anderson said the attack against the 82-year-old stroke victim, "defies belief".
Residential home owners and guests were being warned by the police to be extra vigilant, and to close doors and windows at night, until the man is caught.
Mr Anderson, who is in charge of the inquiry, said: "It defies belief that someone can sink so low as to attack and sexually abuse a helpless old lady who has had a stroke and who is incapable of defending herself."
He added that it had yet to be established how the offender had entered the premises and that the man was very dangerous and must be caught as soon as possible.
The attack took place on the ground floor of the residential care home.
A full search of the area was carried out with police officers and dogs but the man was not found.
Mr Anderson urged anyone who may have seen someone fitting the attacker's description to come forward.
"No-one would want to shield an individual capable of this sort of crime," he said.
The care home's manager declined to comment about the incident other than to say that it was a "sensitive matter".