A bloody shootout has seen 11 cartel members killed - as well as the pet monkey of one of the gang members.
The monkey's owner was among the dead, with the pet - who was wearing a tiny bulletproof vest and camouflage jacket - lying on his chest.
The owner was a young man in his 20s who was said to be part of Mexico 's infamous criminal group La Familia Michoacana.
Since the deaths, a bizarre emotional tribute to the monkey is being shared on social media. It says the animal was called "El Changuito" and was born last year.
It says: "It wasn't your turn, friend. Rest in peace."
Officers found the strange and gruesome scene after a firefight between the cartel members and the authorities in Texcaltitlan, a town 60 miles southwest of the capital Mexico City.
The monkey was found with his arms extended as if holding the owner, who carried him by the shoulders, reports the Sun.
At the time, the small spider monkey was wearing a khaki hooded jacket, a bulletproof vest and a nappy.
The deadly shooting happened when armed members of La Familia Michoacana fought members of the National Guard and of the Attorney General's Office of the State of Mexico.
A total of 10 men were killed at the scene, with another one dying in hospital and a further ten being detained, according to the Mexico Daily Post.
Three are under medical observation as a result of injuries during the shooting.
Five of those detained are reported to be men and three women, reports Infobae, while another is said to be a minor under 15 years of age.
Another adult was taken to hospital, but no further details have been given about them.
The images of the "narco monkey" have gone viral online with many assuming at first that they were fake, but police have now confirmed that they are real.
The monkey's owner is reported to have been part of the regional leader of La Familia Michoacana's security team who fled during a police raid earlier this week.
Police have seized various weapons including ammunition, handguns and 20 assault rifles. They also took the body of the monkey away.
Specialist vets will carry out an autopsy while authorities are said to be investigating the likelihood that the spider monkey - which is a protected species - was illegally trafficked.
