American law enforcement officials arrested two top leaders of the Sinaloa Cartel, one of the most dominant criminal organizations in Mexico, the Justice Department said on Thursday.

The two operatives, Ismael Zambada García and Joaquín Guzmán López, are among the most powerful drug traffickers in Mexico, commanding massive cocaine and fentanyl businesses that move narcotics into the United States, Europe and elsewhere.

Both men were in custody in El Paso, Texas.

The arrests were a victory for the F.B.I., the Drug Enforcement Administration and Homeland Security Investigations, which have been chasing the top ranks of the Sinaloa Cartel for years.

“Fentanyl is the deadliest drug threat our country has ever faced,” Attorney General Merrick Garland said in a statement. “The Justice Department will not rest until every single cartel leader, member, and associate responsible for poisoning our communities is held accountable.”