“He was somebody you don’t want to approach.”

Those who worked with Mr. Heuermann said he was extremely fastidious, impressing some clients while exasperating others with his attention to detail. Some of his neighbors described him as an “average” man whom they wouldn’t think of as “anything but a businessman.” To others, he was someone to avoid — a glowering, towering individual they would see in the front yard of a low-slung, dilapidated house.

“We would cross the street,” said Nicholas Ferchaw, 24, a neighbor. “He was somebody you don’t want to approach.”

Serial killers can have seemingly contradictory personalities, Dr. Bonn said.

“These individuals live compartmentalized lives,” he said, noting that Mr. Heuermann “obviously functioned very highly — had his own architecture firm and picked up his briefcase, got on the train, went into the city every day, went into Manhattan and was able to function.

“But then,” he said, “it’s almost like they flip a switch and just become another individual completely.” *