The United States arrested two of the world’s most wanted drug traffickers on Thursday and accused them of being responsible for the growing presence of fentanyl in the country, where it has devastated communities.

Ismael “El Mayo” Zambada García, 76, and Joaquín Guzmán López, 35, were taken into custody on Thursday, according to the Justice Department. The two men lead the Sinaloa Cartel, one of the biggest criminal organizations in Mexico responsible for a wave of violence that has left a painful scar on Mexico in the past three decades.

Here’s what to know about the arrests and the cartel:

Mr. Zambada García boarded a plane on Thursday with Mr. Guzmán López, who told him they were going to visit investment properties, according to two U.S. law enforcement officials who asked for anonymity to discuss the case. Mr. Zambada García was unaware, the two people said, that the plane was headed for the United States, where officers were waiting to arrest him. Many details of the arrest are unknown.

In the past, even when leaders of the cartels have been imprisoned in Mexico, they have continued to run their criminal organizations. That has prompted the Mexican authorities to allow top leaders to be extradited to the United States. Mr. Zambada García has been indicted at least five times in the United States.

The cartel was founded in the late 1980s by Mr. Zambada García and Joaquín “El Chapo” Guzmán, the father of Mr. Guzmán López, after one of Mexico’s most notorious cartels splintered into rival groups. The two consolidated control over drug routes into the United States and over the next few decades built the cartel into one of Mexico’s most violent and successful, controlling enterprises including drug and human trafficking, and money laundering.