The Mexican government on Thursday began sending to the United States nearly 30 top cartel operatives wanted by the American authorities, including one notorious drug lord that U.S. officials had been seeking to bring to justice for 40 years, according to a statement by the Mexican government.

The handover of so many significant cartel figures to the United States at once was one of the most important efforts by Mexico in the modern history of the drug war to send traffickers across the border to face charges in American federal courts.

The move came as a high-level delegation from Mexico arrived in Washington to meet with senior U.S. officials to hammer out a security agreement at a moment of tension between the two nations. The U.S. government declined to make any immediate public comment, but the Mexican foreign ministry released a statement announcing the release of the cartel figures.

“This action is part of the work of coordination, cooperation and bilateral reciprocity within the framework of respect for the sovereignty of both nations,” the statement said.

Among those being flown to the United States was Rafael Caro Quintero, a founding member of the Sinaloa drug cartel who was convicted in Mexico of having masterminded the 1985 murder of the D.E.A. agent Enrique Camarena, three people familiar with the matter said.

Mexico was also releasing into U.S. custody Miguel Ángel Treviño Morales, an infamously brutal former leader of the Zetas cartel who was captured in Mexico in 2013, the people said.