Colombia's National Penitentiary and Prison Institute (INPEC) confirmed Velasquez died in the early hours of the morning in the National Cancer Institute in the capital Bogota.
The release from INPEC reads: "The National Penitentiary and Prison Institute - INPEC - reports that Mr. Jhon Jairo Velásquez Vásquez ' Popeye', passed away in the early hours of today, at the National Cancer Institute.
"The prisoner had been hospitalized since 31 December 2019, and suffered from stomach cancer."
He has been under arrest since 25 May 2018 on suspicion of extortion and conspiracy to commit a crime.
Velásquez worked as Pablo Escobar's highest-ranking hitman during the 80s and once revealed he was responsible for the mass killing of 110 people with a briefcase bomb in 1989.
The multiple murders were part of a plan to assassinate then presidential candidate César Gaviria who supported extraditing Colombian drug dealers to the US to face prosecution.
The politician was supposed to be travelling onboard a domestic Avianca airlines flight from Bogota to Cali, however, his security prevented him from boarding.
Velasquez later described the method as a 'work of art', telling the Daily Beast: "Castano [Carlos Castaño Gil - another member of the cartel] deceived a young man and told him that he should open his briefcase in the air when the plane took off and began to climb.
"He told him that the briefcase held a tape recorder and that when he opened it the device would be activated to grab the conversations of certain passengers.
"That there were some Americans aboard, and the mafia needed to know about an extradition case they were carrying out.
"The dupe had already held a briefcase with a complex recorder in his hands and saw how it worked.
"Castano had already given him several examples, to deceive him more easily. When the briefcase opened in the air, the bomb was activated."Featured Image Credit: PA