MEDELLÍN, Colombia — A former guerrilla peace negotiator has been arrested on drug trafficking charges in Colombia, after government officials said he and associates had returned to the drug trade his group had vowed to give up.

The arrest of Seusis Hernández Solarte was a surprise turn of events for the FARC, the former rebel organization that has refashioned itself as a political party called the Common Alternative Revolutionary Force.

In signing a peace deal in 2016, its leaders — all wanted as drug traffickers in the United States — were promised immunity for past crimes and, in return, they renounced future ones.

But on Monday, the Colombian government said that Mr. Hernández, whose nom de guerre was Jesús Santrich, had quickly returned to the drug business. Officials said that starting last June, just months after the peace deal was signed, Mr. Hernández had sought to export roughly 10 tons of cocaine to the United States, whose value they estimated was as much as $320 million.