Mexico City police are hunting a serial killer thought to be behind the unusual murders of as many as 15 elderly women.
The suspect - nicknamed "Mataviejitas" or the "Little Old Lady Killer" - is thought to be a man dressed in women's clothes or a well-built woman.
The killer is said to have entered the victims' homes by winning their trust - possibly by posing as a health worker - before beating and strangling them.
The Mexican capital has one of the highest crime rates in the world.
Witnesses in the case of an 85-year-old widow killed last month said they saw a large woman in a red blouse - who may also have been a man disguised as a woman - leaving the victim's house.
Mexico City chief prosecutor Bernardo Batiz said the killer was "brilliantly clever", acting alone and winning the confidence of old people.
Three of the last four victims of the killer owned a print of an 18th century painting, Boy in Red Waistcoat, by French artist Jean-Baptiste Greuze.
Prosecutors say this bizarre link may be mere coincidence.
A government body has distributed thousands of leaflets warning the elderly over the killings.
Mexican authorities have been criticised by the United Nations for failing to solve cases of violence against women.
The unsolved murders of hundreds of women in the border city of Ciudad Juarez over the last decade have attracted international attention.