Black-clad gunman Devin ​Patrick ​Kelley executed some of his victims inside a Texas church by shooting them in the head, ​officials who viewed video of the massacre said Wednesday.

Gruesome video footage from inside First Baptist Church in Sutherland Springs shows Kelley firing at worshipers for about seven minutes, investigators told ​t​he New York Times.

He shot some victims — including kids — in the head with his semiautomatic assault rifle, the footage shows.

The massacre left 26 dead and 20 others wounded.

On Tuesday, survivor Joaquin Ramirez told The Post that Kelley intentionally shot children who were crying during his killing spree.