Mexico’s Foreign Ministry said in a statement it extended the invitation through a diplomatic note to the U.S. Embassy in Mexico.
“The Mexican government reiterates its commitment to investigate the facts, in order to … offer justice to the affected families,” the ministry said.
The U.S. agents must work in coordination with their Mexican counterparts and will not be armed, it added.
The FBI did not immediately respond to a request for comment.
On Monday, gunmen killed three women and six children from a breakaway Mormon community in northern Mexico, prompting U.S. President Donald Trump to offer Mexican President Andres Manuel Lopez Obrador help in wiping out drug gangs he blamed for the ambush.