SANTA FE, Tex. — The exact reasons a teenage gunman shot his fellow students and teachers here at Santa Fe High School remain a mystery. His model for carrying it out is more clear.

The 17-year-old junior wore a black trench coat and fired a sawed-off shotgun, the same attire and weaponry used by the two gunmen who killed a dozen students and a teacher at Columbine High School in Colorado in 1999.

He wore a T-shirt with the phrase “Born to Kill” on it in bold, similar in design to those worn by the Columbine attackers, which read “Wrath” and “Natural Selection.”

His crude arsenal included canisters of carbon-dioxide gas and Molotov cocktails, two types of explosives used by the Columbine gunmen.

The picture he had posted of his trench coat on Facebook showed a small red-star medallion with the Communist hammer-and-sickle on the collar, the same type of button a Columbine gunman attached to his boot.

It was not the first time a high school suddenly engulfed in gunfire and death found itself looking for clues in the random symbology of a nearly 20-year-old mass shooting that has become, it seems, the standard by which youthful gunmen across America have come to measure themselves.

The 20-year-old attacker who killed 26 people at Sandy Hook Elementary School in Connecticut in 2012 had compiled an enormous mass-murder spreadsheet and materials on the Columbine attackers on his computer, including what appeared to be a complete copy of the official Columbine investigation.