Authorities said Friday that two former rebel leaders of the Revolutionary Armed Forces of Colombia have abandoned their camps, raising fears they could abandon a fragile peace process that ended five decades of internal conflict.

Colombia's lead prosecutor Nestor Humberto Martinez said that the whereabouts of Luciano Marin, who is more commonly known as Ivan Marquez, are unknown.

Office of National Protection director Diego Mora said that Henry Castellanos sent his bodyguards home to the capital of Bogota earlier this week and his location is also a mystery.

The two former commanders helped negotiate a peace deal with the government in 2016 under which the rebel group known as the FARC was promised 10 seats in Colombia's Congress and reduced sentences in exchange for laying down their weapons and not participating in future crimes.

But U.S. officials are investigating Marquez in connection with alleged drug trafficking schemes that took place after the peace deal was signed.

In April Colombian police arrested a close ally of Marquez, former rebel leader Jesus Santrich, at his home in Bogota on a request from a New York court that charged Santrich with conspiring to export cocaine to the United States.

Following Santrich's arrest Marquez fled Colombia's capital city for a remote camp in the south of the country where rebels are working on agricultural projects.

From the camp, Marquez resigned from a seat in Colombia's Senate that he had been granted by the peace deal, arguing that he had no "legal guarantees" to participate in Colombian politics.

Mora said he had not heard from Marquez since early August, when the former rebel leader sent him a letter in which he renounced his government-assigned security detail. The bodyguards promptly returned home and Marquez has not been heard from since.

Former FARC leaders who have now become politicians, have suggested that Marquez is still in southern Colombia, hiding from military operations that could put his life at risk. But Colombia's army said this week that it is investigating whether Marquez and another former rebel leader have escaped to neighboring Venezuela.

On Friday, Colombia's lead prosecutor said Marquez is not wanted by Colombian authorities on drug trafficking charges.

The FARC said that Marquez and former rebel leader Henry Castellanos, did not show up at the organization's annual conference, which started on Friday.

The concern about rebel leader abandoning the peace process comes as Colombian President Ivan Duque, a fierce critic of the peace deal, said he would welcome the UN renewing a mission to track implementation of the deal.

"We believe international supervision is very important to help us implement the rule of law" Duque said on Friday. "And to allow guerrilla fighters to make a successful transition," to civilian life.