BOSTON (AP) — Michael McDermott was often surly to neighbors and co-workers, yet embraced the nickname "Mucko" given him by nieces and nephews who couldn't pronounce his first name.

To his acquaintances, the former U.S. Navy electrician and software engineer was known to keep to himself. He was recognized more for his bearish 6-foot-2, 260-pound frame and bushy, black beard than any personality traits.

Today, much more is known about him.

Prosecutors say McDermott, 42, apparently angry over a demand by the Internal Revenue Service to garnishee his wages for back taxes, carried an arsenal of weapons into his workplace and opened fire Tuesday, killing seven co-workers.

"Of all the people that I thought could have done this, it was him," said Mike Stanley, a team projects leader at Edgewater Technology in Wakefield, where the slayings took place. He described McDermott as "strange" and "quirky."

McDermott has no known criminal record. He served eight years aboard the nuclear submarine USS Narwhal and was divorced with no children.

A co-worker who spoke under the condition of anonymity said McDermott went by the nickname "Mucko" — something his nieces and nephews came up with when they couldn't pronounce Michael. He even had it on his car's license plate.

McDermott had worked at Edgewater as a software tester since March, company officials said. He usually dressed in flannel shirts, jeans and sneakers.

Jonathan Oldham, a neighbor in Weymouth where McDermott lived in a rented condominium until the end of October, said McDermott kept to himself.

"He was very quiet and didn't say much," Oldham said. "I just passed him in the hallway and said 'hi' to him."

The condo's owner, Saburo Imura, told The Washington Post that McDermott shared a lease there for 10 years.

Imura said McDermott and his wife, who later divorced, had been chronically late paying their rent over the years, and McDermott skipped out without paying for September and October, at $840 a month.

Kevin Forzese, of Haverhill, said he met McDermott in October, when he offered Forzese $100 to help him move into the apartment below. He said McDermott's only regular visitor was a girlfriend.

Forzese said McDermott never mentioned his work: "I talked to him about money and he said he was doing really well."

And while McDermott was mostly surly and sarcastic, he nonetheless remembered to send Forzese a holiday card.

On Christmas morning, Forzese noticed a striking change in McDermott's demeanor as the two met in the stairwell of their apartment building.

"It was the first time I'd ever seen him smiley, cheery. I was just shocked," Forzese said.

Forzese thanked him for the Christmas card and McDermott said he planned to visit his mother for the holiday.

McDermott had told Forzese he collected antique guns, and while Forzese found McDermott "had an oddity about him," he was shocked to hear his neighbor was a suspect in a mass killing.