Madrid - Spanish police have detained four men in connection with the killing of Colombian drug lord Leonidas Vargas at a Madrid hospital on January 8, police sources said Friday.

The three Colombians and one Romanian citizen were held in the Madrid region late Thursday.

Police seized pistols, ammunition, bulletproof vests and other materials.

Vargas, 59, was shot dead in his hospital bed while undergoing treatment for lung and heart problems.

A suspected hired killer walked into the room, asked Vargas' fellow patient whether the sleeping man was the Colombian, told him to turn his back and to keep quiet, and fired four shots into his victim.

Another man was believed to have kept watch outside the door. The killers fled before the murder was discovered.

Vargas, alias El Viejo (The Old One), by his real name Jose Antonio Ortiz Mora, had been freed on bail for health reasons while awaiting trial in Spain.

He was detained in 2006 at the entrance to a Madrid hotel after police seized 500 kilograms of cocaine in the eastern port of Valencia.

His murder was believed to be a settlement of accounts between drug traffickers.

Vargas is believed to have cooperated with the late Pablo Escobar, Colombia's most famous drug lord.

In the 1990s, he became known as the "king of Caqueta," a southern region where he allegedly ran extensive drug laboratories.

In 1995, Vargas was sentenced to 19 years in prison for drug trafficking and to 25 more years for homicide, illegal possession of weapons and illicit earnings.

In practice, he served only until 2001 in a high-security prison near Medellin.

The Colombian government seized from Vargas properties worth 29 million dollars.

Vargas' brother and his girlfriend were shot dead in Colombia a few days after his killing in Madrid. (dpa)