MEXICO CITY - President Andrés Manuel López Obrador on Wednesday rejected U.S. suggestions he get more aggressive against violence after this week's massacre of fundamentalist Mormons in northern Mexico, saying the war-on-drugs approach has been a "disaster" in the past.

Authorities also noted that the bullets used in the killings of three women and six children in the northern state of Sonora on Monday were manufactured in the United States.

The slaying Monday of the members of the extended LeBaron family - dual U.S.-Mexican citizens - has raised pressure on López Obrador's leftist government, which has pledged to use social programs to address the root causes of violence.

"It's unfortunate, sad, because children died. This is painful," López Obrador told reporters Wednesday. "But trying to resolve this problem by declaring a war? In our country, it's been shown that this doesn't work. This was a disaster."

He was referring to the U.S.-backed offensive against drug groups launched in 2006, with the deployment of the Mexican army to battle organized-crime groups. Around 200,000 people have died in violence related to the conflict.

Mexico continues to work closely with the U.S. government in pursuing drug kingpins. President Donald Trump, however, suggested a more aggressive approach after news of the killings broke Tuesday, tweeting that "this is the time for Mexico, with the help of the United States, to wage WAR on the drug cartels and wipe them off the face of the earth. We merely await a call from your great new president!"

Other American politicians also expressed alarm at the violence surging south of the border.

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Sen. Tom Cotton, R-Ark., said "the Mexican government can't handle this" and that the United States might have to step in.

The Mexican leader's strategy "may work in a children's fairy tale," Cotton told Fox News. "But in the real world . . . the only thing that can counteract bullets is more and bigger bullets."

López Obrador responded that Cotton's comments reflected "his vision."

"We respect it, but we don't share it," the president said.

Mexican authorities noted pointedly on Wednesday that the cartridges recovered from the attack were .223 caliber and produced by Remington, a U.S. firm. Those are typically used in M-16 and AR-15 assault rifles.

Mexico's public security secretary said 70 percent of all the weapons tied to a crime in Mexico were smuggled from the United States. "That's why the cooperation of the U.S. government will be fundamental to get good results," Secretary Alfonso Durazo said.

There was no immediate response to emails seeking comment from Remington. Mexico has extremely strict laws restricting the sale and possession of firearms; there's only one gun store in the country - and it's on an army base.

Foreign Minister Marcelo Ebrard said Wednesday that all details of the investigation would be shared with U.S. authorities, and Mexico was open to considering U.S. offers of assistance. "The extent of the participation by the FBI or other U.S. institutions will depend on what the attorney general's office determines," he said.

Mexican officials said Wednesday that the attack might have been linked to a fight between local groups connected to the Juarez and Sinaloa cartels. The groups - known as La Linea and Los Salazar - had clashed a day before the families were killed, according to Gen. Homero Mendoza, the Defense Ministry's chief of staff.

Mendoza told reporters that La Linea sent gunmen to the Bavispe area to defend its territory. The three attacks on the LeBaron family occurred near that municipality, close to the border of Chihuahua and Sonora states - a key corridor for sending drugs to the United States.

"We assume this cell that was sent to prevent any incursion by Los Salazar is what is behind the aggression against the family," Mendoza said.

He said the gunmen might have mistaken the families' SUVs for vehicles belonging to its rivals.

Family members have protested that security forces took hours to reach the scene of the crime, forcing them to conduct their own search for victims and survivors. Some said the Mexican government should accept U.S. offers of assistance.

"We'll accept help from aliens, if it comes to that," family spokesman Julian LeBaron told El Universal.

Mendoza said the attacks occurred at 9:40 and 11 a.m. Monday, but authorities didn't learn about them until 2:30 p.m., because area residents were too frightened to approach the sites. The military didn't arrive at the scene until 6:15 p.m., he said.

"We had very few personnel" in the area, the general said. He noted that it had not been identified as a conflict zone. He said the military and National Guard were rushing more forces to the region.

The attacks, the latest in a series to rock Mexico, have brought more scrutiny of López Obrador's new approach against organized crime. The president, who took office last December, has created a 70,000-member National Guard, but it has been unable to halt the violence. Mexico is likely to set a record for homicides this year.

Critics say the government hasn't used the new force strategically to contain violence - troops have instead been spread around the country to handle a variety of tasks, including detaining unauthorized migrants heading for the U.S. border.

Alejandro Hope, a security analyst, noted that the National Guard had 3,799 troops in Mexico City as of Oct. 14. It had just 4,126 troops in Sonora and Chihuahua, which represent 21 percent of the national territory.

"If this force serves any purpose, it should be for territorial control, to ensure a presence of the state where it's almost impossible to add police," Hope wrote in El Universal.

Durazo said authorities investigated a suspect arrested Tuesday in Sonora with multiple weapons and two hostages, but it appeared he was "not connected to the aggression against the LeBaron family."

This article was written by Mary Beth Sheridan, a reporter for The Washington Post.