Americans who leave our country to reside in another are outside the protection of our government in most ways. But the sheer horror of what happened in Mexico last week — nine women and children massacred, apparently by drug cartel terrorists — cries out for some meaningful reaction by Washington.

A relatively large group of Americans, apparently disenchanted with the Church of Jesus Christ of Latter Day Saints but still considering themselves Mormons, moved to an area near La Mora, Mexico, a few years ago. It had been populated by other U.S. Mormons nearly a century ago.

For years, the Mormon enclave there, with its members still considered U.S. citizens, had gotten along well. That changed Monday.

On that day, gunmen in two separate ambushes stopped vehicles filled with Mormon women and children. Nine of them were killed, some by gunfire and some in fire that engulfed one of their SUVs.

Initially, Mexican authorities blamed the massacre on a mistake made by one drug cartel. Its gunmen had meant to attack rivals, but killed the American women and children by mistake, it was said. The fact that some children managed to escape the slaughter was cited as evidence of that theory.

But whatever the killers’ motive, they murdered nine women and children – U.S. citizens.

U.S. investigative help already has been offered to the Mexican government. It should be followed up with a discrete but firm demand that the killers be tracked down and brought to justice.

To that end, President Donald Trump should offer other U.S. resources, including armed U.S. law enforcement personnel and, if necessary, troops.

This was a truly horrific crime that must be punished harshly, in part to send a message to terrorists everywhere that when Americans are their victims, reprisal will be swift, certain and overwhelming.