Law enforcement officials here and in Wichita, a conservative town that has been a focal point of tense abortion debate in large part because of Dr. Tiller’s clinic, gave little sense of whether they had previously viewed Mr. Roeder as a concern. After he was taken into custody, they indicated that they were only beginning to delve into his past and his associations.

Image Scott Roeder after an arrest in 1998. Credit... Shawnee County Department of Corrections

Still, as Mr. Roeder’s relatives and others who had come into contact with him over the years began looking backward, they said they now saw some signs that might have hinted at more serious trouble ahead. For more than 10 years, Mr. Roeder had been linked, at various times and in varying degrees, to the Freemen, a group that rejected federal authority and the banking system, and to people who believe that the killing of abortion providers was justified by the abortions it prevented.

In 2007, someone identifying himself as Scott Roeder posted a message on the Web site of Operation Rescue, a group based in Wichita that had devoted much of its effort to blocking Dr. Tiller from performing late-term abortions. The posting read, in part: “Tiller is the concentration camp ‘Mengele’ of our day and needs to be stopped before he and those who protect him bring judgment upon our nation.”

The leader of Operation Rescue, who denounced the shooting of Dr. Tiller, said he had never met Mr. Roeder, who was not a contributor, volunteer or regular member. And the head of the Kansas Coalition for Life, whose volunteers spent hours outside Dr. Tiller’s clinic each week trying to sway patients from abortions, said he had never met Mr. Roeder, though he recalled receiving three phone calls out of the blue from him last August.

Years earlier, Mr. Roeder belonged to a Kansas group known as the Patriot Movement, a citizens’ militia which, according to a fellow member, Morris Wilson, 70, aimed to “kick Uncle Sam in the shins” by bucking rules like mounting license plates on cars. “He didn’t like taxation and overregulation,” Mr. Wilson recalled, adding that Mr. Roeder had outspoken views against abortion.

“He was trying to get people aware of what was going on, and put these guys out of business,” he said. “But I never seen a temper.”