Via American Action News:

Mexican authorities have arrested a suspect behind the massacre of nine American citizens – including six children. 

Cornered in a bulletproof SUV, he reportedly had two hostages with him, bound and gaged.

Fox News' Brie Stimson has more:

The victims' convoy came under fire during the day in an ambush by gunmen believed to be affiliated with a drug cartel in Mexico.

The Agency for Criminal Investigation said in a statement on Facebook the suspect was found in the town of Agua Prieta, right across the border from Douglas, Arizona. The suspect was holding two hostages who were bound and gagged inside a vehicle.

...

Officials have said that the gunmen may have mistaken the group's large SUVs for those of a rival gang amid a vicious turf war.

Eight young children - including babies - survived the attack by hiding in the brush and even though they were wounded some walked miles to get help. Five badly wounded children were transported to hospitals in Arizona for treatment.