MEXICO CITY — The leader of one of Mexico’s most violent and feared drug organizations, the Zetas, was captured Monday in a city near the Texas border, an emphatic retort from the new government to questions over whether it would go after top organized crime leaders.

The man, Miguel Ángel Treviño Morales, 40, who goes by the nickname Z-40 and is one of the most wanted people on both sides of the border, was detained by Mexican marines Monday morning, Mexican officials said at a news conference Monday night.

He was detained about 3:45 a.m., without a shot being fired, as he traveled in a pickup truck near Nuevo Laredo, opposite Laredo, Texas, with two other men who were also detained, the officials said, adding that the marines seized $2 million in cash and weapons.

Mr. Treviño was ranked among the most ruthless crime bosses, wanted for murder, organized crime, and torture; he has been linked to the killing and disappearance of 265 migrants in northeastern Mexico, including 72 found dead in August 2010.

He also faces drug and gun charges in the United States, which has offered a $5 million reward for information leading to his capture.

Eduardo Sánchez, the spokesman on security matters for the Mexican government, declined to say what role the United States played in the capture, though American law enforcement tips have often been behind high-profile arrests.

An American law enforcement official declined to provide details, deferring to the Mexican government announcement of the arrest, which was first reported by The Dallas Morning News on its Web site.

The Zetas operate primarily in Mexico, but their drug trafficking and organized crime violence have spread to other countries, and they have been known to recruit members in Texas and even to launder money through the quarter-horse industry in the United States.

Started by former soldiers and once the enforcement arm of another large cartel, the gang is known in Mexico for its brutality, and its members’ calling card is often beheaded victims, body parts on highways and bodies hanged from bridges.

Mr. Treviño is the highest-ranking and most-sought-after drug capo arrested by the government of President Enrique Peña Nieto of Mexico, whose aides had questioned the so-called kingpin strategy of his predecessor, which had emphasized high-profile arrests. The leadership voids, battles for turf and confrontations with Mexican forces all sent violence soaring in the past several years, with tens of thousands dead or missing.

The new government had scoffed at the deep level of involvement of American law enforcement and security agencies in Mexico and placed new limits on their access, causing some American officials and analysts to wonder whether it would be deeply committed to confronting the drug gangs. Michele M. Leonhart, the head of the Drug Enforcement Administration, visited Mexico on Friday, and the leaders of Mexico’s army and navy are visiting Washington to forge closer ties.

The arrest will probably give doubters some hope, experts said.

“The success of the effort is likely to help build trust after a period of rocky relations on public security issues,” said Andrew Selee, a Mexico scholar at the Woodrow Wilson Center in Washington.

Some analysts said the arrest could lead to further fragmentation of the gangs, which would reduce their ability to threaten state authority but might uncork further waves of violence.

“This takedown will boost Peña Nieto several points in the polls, even as he has spurned talking about violence and the narco war,” said George W. Grayson, a professor at the College of William and Mary who has written extensively on the drug gangs. “They fragment into ‘cartelitos,’ which, while dangerous, do not pose a threat to state security.”

Mr. Treviño had been the second in command until the Zetas leader Heriberto Lazcano Lazcano was killed in a battle with Mexican marines in October.

His body was carted off by armed men from a funeral home shortly afterward in an episode that turned triumph into embarrassment for Mexico’s president at the time, Felipe Calderón, whose tenure was marked by the killing or arrest of several cartel leaders except the most elusive: Joaquín Guzmán, known as El Chapo, the leader of the Sinaloa Cartel, considered the largest and most powerful supplier of cocaine to the United States.

At his peak, Mr. Treviño was widely feared and credited with helping to give the Zetas gang its reputation while transforming it from a hit squad to a mushrooming transnational criminal organization.

“He had all of Mexico and a lot of Central America under his tentacles,” said Art Fontes, a recently retired F.B.I. agent who was assigned to Mexico until late last year and is now a security consultant. “He was feared everywhere he went.”

In one of the organization’s bolder moves across the border, Mr. Treviño used a brother in the United States to launder tens of millions of dollars in drug proceeds by buying and selling expensive American quarter horses. José Treviño, the kingpin’s older brother, was convicted this year of running the business, whose operations were first reported by The New York Times.

But a number of drug war analysts have said that Mr. Guzmán’s cartel, an older, more established organization less prone to shocking violence, was beginning to overtake the Zetas, the younger, less disciplined outfit that branched out more into extortion, kidnapping and migrant smuggling.

Mexican law enforcement has arrested some of the Zetas’ most important leaders, including several of them just before Mr. Calderón’s term ended in December.

Insight Crime, a news and analysis Web site that closely tracks drug crime in the Americas, has reported that a Zetas splinter group called Los Legionarios emerged last year “with the express purpose of waging war against Z-40 and his organization.”