By Jeremy McDermott BBC correspondent in Medellin

The assassination of Colombian Archbishop Isaias Duarte Cancino had all the hallmarks of a contract killing.

The list of suspects includes drugs traffickers, allegedly corrupt politicians and Marxist rebels.

What is going on? Who are the dark forces that are trying to destabilise this poor country?

Monsignor Alberto Giraldo

Authorities have no leads, but as the country suffers under a wave of attacks by Marxist guerrillas after the breakdown of peace talks last month, police were quick to pin the blame on the rebels.

"We believe this was the work of the subversives, but we never had first-hand information that he had received threats," said police general Heliodoro Alfonso Roa.

The rebels might have had reason to want to kill the archbishop, as he had been an outspoken critic of their methods.

He even excommunicated a group from the country's second largest rebel group, the National Liberation Army (ELN), after 150 guerrillas burst into a Sunday church service in May 1999 and kidnapped 150 worshippers, including the priest.

Candidates denounced

Monsignor Duarte might have also provoked the hatred of the larger Revolutionary Armed Forces of Colombia (FARC), after he was mentioned in affectionate terms in the biography of their most hated enemy, the right wing paramilitary warlord, Carlos Castano.

In the book My Confession, Castano described the archbishop as "a friend".

But Monsignor Duarte's most recent controversy was last month when he denounced candidates standing for congress in the elections that were held on 10 March.

The archbishop had criticised rebel groups

He insisted they were financed by drugs traffickers, prompting speculation that corrupt politicians or drug lords were behind the murder.

The police are also coming under pressure, particularly after General Alfonso Roa's comment that he had received no information of threats, as this was directly refuted by a spokesman for the archbishop's office in Cali, Father Gersain Paz.

"The priest of Buen Pastor (the church where the archbishop was killed) saw the suspects at 1600 (2300 GMT Saturday), he called the police and asked for security to be reinforced.

"I don't know why they did not provide sufficient security, which left him to die without any protection," said a distraught Father Paz.

The head of the Catholic Church in Colombia, Monsignor Alberto Giraldo, was visibly crushed by the news.

He asked: "What's going on? Who are the dark forces that are trying to destabilise this poor country?"