At one point, Mr. Saipov made his way to Fort Myers, Fla., where he met a fellow Uzbek immigrant, Kobiljon Matkarov, 37. Mr. Saipov was working as a truck driver at the time.

“He was a very good person when I knew him,” he said. “He liked the U.S. He seemed very lucky, and all the time he was happy and talking like everything is O.K. He did not seem like a terrorist, but I did not know him from the inside.”

Image Sayfullo Saipov Credit... St. Charles County Department of Corrections

As investigators began on Tuesday to look into Mr. Saipov’s history, it became clear that he had been on the radar of federal authorities. Three officials said he had come to their attention as a result of an unrelated investigation, but it was not clear whether that was because he was a friend, an associate or a family member of someone under scrutiny or because he had been the focus of an investigation.

Over the last two years, a terrorism investigation by the F.B.I., the Department of Homeland Security, the New York Police Department and federal prosecutors in Brooklyn resulted in charges against five men from Uzbekistan and one from Kazakhstan of providing material support to ISIS. Several of the men have pleaded guilty. It is unclear whether Mr. Saipov was connected with that investigation.