Nearly 100 minutes before former President Donald J. Trump took the stage in Butler, Pa., a local countersniper who was part of the broader security detail let his colleagues know his shift was ending.

“Guys I am out. Be safe,” he texted to a group of colleagues at 4:19 p.m. on July 13. He exited the second floor of a warehouse that overlooked the campaign rally site, leaving two other countersnipers behind.

Outside, the officer noticed a young man with long stringy hair sitting on a picnic table near the warehouse. So at 4:26 p.m., he texted his colleagues about the man, who was outside the fenced area of the Butler Fair Show grounds where Mr. Trump was to appear. He said that the person would have seen him come out with his rifle and “knows you guys are up there.”

The countersniper who sent the texts confirmed to The New York Times that the individual he saw was later identified as the gunman.

By 5:10 p.m., the young man was no longer on the picnic table. He was right below the countersnipers, who were upstairs in a warehouse owned by AGR International. One of the countersnipers took pictures of him, according to a law enforcement after-action report, which along with the texts from the Beaver County Emergency Services Unit was provided to The Times by the office of Senator Charles E. Grassley, Republican of Iowa.