"Merry Christmas! ps. Santa, please bring ammo," US Representative Thomas Massie of Kentucky wrote on Twitter.
Ethan Crumbley, 15, on Tuesday carried out the deadliest US school shooting this year, the latest in a decades-long series of mass shootings at US schools. His parents were arrested on Saturday in connection with the slayings.
Massie, who represents a solidly Republican district, posted the picture of himself and six others holding firearms resembling an M60 machine gun, AR-15 semi-automatic rifle and a Thompson submachine gun.
Some semi-automatic weapons are made to look nearly identical to fully automatic weapons like machine guns. Under US law, weapons like machine guns are restricted to the military, law enforcement and civilians who have obtained special licenses for weapons made before May 1986.
Jonathan van Norman, a campaign manager for Massie, did not immediately reply to a request for comment via Twitter.
Democratic US Representative John Yarmuth, who chairs the US House of Representatives Budget Committee, condemned his fellow Kentuckian's post.
"I’m old enough to remember Republicans screaming that it was insensitive to try to protect people from gun violence after a tragedy," Yarmuth wrote on Twitter, apparently referring to calls for gun control laws.
"I promise not everyone in Kentucky is an insensitive asshole," he added. The shooting in Oxford, Michigan - in which four teenagers were killed and a teacher and six other students were wounded - was the latest in a string of sometimes-deadlier incidents that have prompted fierce debates over school safety, gun control and gun rights.