The U.S. Coast Guard lieutenant spent hours on end planning a wide-scale domestic terrorist attack, even logging in at his work computer on the job at headquarters to study the manifestos and heinous paths of mass shooters, prosecutors say. He researched how to carry out sniper attacks, they contend, and whether rifle scopes were illegal. And all the while, investigators assert, he was amassing a cache of weapons as he ruminated about attacks on politicians and journalists.

But Christopher P. Hasson was not an isolated figure, according to a contractor who worked with him. The 49-year-old lieutenant with more than two decades in the Coast Guard was part of a project to replace some aging cutters in the fleet, tasks that regularly required interacting with civilians and military officials at meetings and on travel.

“I don’t remember him saying anything that was crazy,” said Adam Stolzberg, a contractor who worked at headquarters and was in meetings with Hasson a couple of times a month. Politics never came up, Stolzberg said.