The FBI has offered assistance to the Mexican government as authorities investigate the brazen killing of nine American citizens by drug cartel gunmen in northern Mexico on Monday, officials told Fox News.

It was unclear of Mexican authorities have taken up the offer. It came hours after Mexico rejected President Trump's overture to "wage war" on the country's drug cartels in response to the brutal slayings.

“If Mexico needs or requests help in cleaning out these monsters, the United States stands ready, willing & able to get involved and do the job quickly and effectively,” Trump tweeted Tuesday. “The great new President of Mexico has made this a big issue, but the cartels have become so large and powerful that you sometimes need an army to defeat an army!”

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Mexican President Andrés Manuel López Obrador thanked Trump but declined the help, saying, "The worst thing you can have is war."

"We declared war, and it didn't work," Lopez Obrador said, referring to the policies of previous administrations. "That is not an option."

Officials said at least six children, including 8-month-old twins, and three women living in a faith-based community called La Mora in the Mexican state of Sonora were fatally shot after their convoy came under assault from gunman believed to be affiliated with drug cartels.

One woman was killed as she exited a vehicle to wave her hands and demonstrate she wasn't a threat, prosecutors said.

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Six more children were wounded and one was missing following the ambush. All nine victims had dual American-Mexican citizenship. They were on their way to visit relatives in Mexico and one was headed to an airport in Phoneix to meet her husband.

The victims are believed to be members of the extended LeBaron family, who live in La Mora, as part of an offshoot of the Church of Jesus Christ of Latter-day Saints around 70 miles south of Douglas, Ariz.

Mexican Security Secretary Alfonso Durazo said the gunmen possibly mistook the convoy for that of a rival drug gang.

The area where the bloodshed took place is a remote, mountainous region where the Sinaloa cartel has been engaged in a bloody turf war. Many of Mexico's drug cartels have become more brazen in recent years, killing women and children in battles with rival groups.

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"Lately it's getting worse. This is a whole new level," said Taylor Langford, a relative of the dead who splits his time between the Mexican community and his home in the Salt Lake City suburb of Herriman, Utah.

The killings have put a spotlight on López Obrador's crime-fighting strategy, dubbed "'hugs, not bullets."

Last month, Mexican forces released a son of imprisoned drug lord Joaquin "El Chapo" Guzman after cartel gunmen launched a counterattack in Culiacan, Sinaloa.

López Obrador said the decision stopped the gunmen from targeting innocent civilians and ultimately saved lives

Fox News' Julia Musto, Robert Gearty and The Associated Press contributed to this report.