“He was very cordial, very charming, you would never know that something was wrong,’’ said Lonni Heisman, his mother. He frequently told her he loved her, and had recently visited to help her prepare for a move. “I’m still in shock and I can’t understand how he could have hid this,’’ she said. “He had everything going for him. He was in perfect health. He was handsome, smart, a good person. I’ll never understand it.’’

Neither his mother, sister, nor the roommates from whom he sought forgiveness in the hours before he died had any idea he was about to kill himself. They and others have been groping for answers to why he did it and in such a public way, on such a holy day.

He told them he was working on a history of the Norman conquest of England, cloistered in a cramped apartment he shared in Somerville. They knew the clean-shaven young man from suburban New Jersey, who always called his elderly godmother on her birthday and once donated $200 to Harvard Hillel for sponsoring services at Memorial Church, to be intensely committed to his work.

Over the years, as he became more immersed in his work, often laboring over it 12 hours a day, Heisman shared bits with friends and family but never elaborated on the extent of his nihilism — his hardened view that life is vapid and nonsensical, that values are pretense, that the “unreasoned conviction in the rightness of life over death is like a god or a mass delusion.’’

“Every word, every thought, and every emotion come back to one core problem: life is meaningless,’’ he wrote. “The experiment in nihilism is to seek out and expose every illusion and every myth, wherever it may lead, no matter what, even if it kills us.’’

At the end of his note, a dense, scholarly work with 1,433 footnotes, a 20-page bibliography, and more than 1,700 references to God and 200 references to the German philosopher Friedrich Nietzsche, Heisman sums up his experiment:

But if the 1,905-page suicide note he left is to be believed — a work he spent five years honing and that his family and others received in a posthumous e-mail after his suicide last Saturday morning on Yom Kippur — Heisman took his life as part of a philosophical exploration he called “an experiment in nihilism.’’

In the end, no one really knows what led Mitchell Heisman, an erudite, wry, handsome 35-year-old, to walk into Harvard Yard on the holiest day in his faith and fire one shot from a silver revolver into his right temple, on the top step of Memorial Church, where hundreds gathered to observe the Jewish Day of Atonement.

She said he was a gregarious child who grew introverted after his father, an engineer, died of a heart attack when Mitchell was 12 years old. As he got older, he became increasingly bookish and went on to study psychology at the University at Albany in New York, where he seemed shy to friends and spent much of his time reading.

After college, Heisman worked at bookstores, including the Strand in Manhattan, enabling him to amass a library of thousands of books. About five years ago, he moved to Somerville to focus on writing and be near major university libraries.

He led a Spartan existence, subsisting on microwave meals, chicken wings, and energy bars, and surviving mainly on money left to him after his father’s death. He was tall, with dark eyes, and dated when he needed a break from his solitude, rarely having trouble attracting women. But he broke off the relationships quickly, saying he was too busy writing a book.

To help him concentrate, Heisman often listened to a constant loop of Bach’s “Well-Tempered Clavier,’’ which he felt synthesized the mind’s competing strains of emotion and reason, went to a gym daily, and took Ritalin, which his mother thinks may have induced depression and led to his suicide.

One of his longtime roommates, David Barnes, described Heisman as quiet and considerate, never angry. He engaged in conversation by asking questions; when he spoke he often gave deliberate, lengthy responses. “He could get intense talking about his book,’’ Barnes said. “There was definitely a lot of emotion pent up in this project.’’

Barnes and relatives said Heisman bought the gun, a .38-caliber pistol, three years ago, though they don’t know where, and they believe he had only one purpose for it: to commit suicide when he finished his book.

“He wasn’t going anywhere dangerous; he wasn’t paranoid; he wasn’t worried about anyone hurting him or breaking in,’’ Barnes said. “I couldn’t imagine him buying a gun for any other reason.’’

A month ago, as he began wrapping up his writing, he asked Barnes if he would be a witness to the signing of his will. Barnes thought it was because he cared so much about his book and wanted to ensure it would be taken care of in case something happened.

Two days before his suicide, Heisman seemed elated. He told his roommates he had finished the book. He spent the next day at the post office, buying stamps and preparing packages for friends and family, with the book on CDs.