Cynthia Wachenheim
Sometime before 3:25 p.m. Wednesday, Cynthia Wachenheim, a lawyer who was on child-care leave from her job, wrote out a note. On lined notebook paper, it ran for 13 pages.

According to a law enforcement official who has seen the note, she wrote that her infant son, Keston Bacharach, had previously taken a few tumbles, including "two shameful incidents," a fall from a Gymini play set onto the wood floor when she walked out of the room for five minutes, and off a bed. She blamed herself, and was convinced that those falls had led to a series of concussions and seizures that aggravated or contributed to maladies that would harm him for the rest of his life.

Her friends, family members and pediatrician did not believe her, she wrote. But she noticed changes in the baby - changes that only a mother who spends all day with her child would notice. For instance, she wrote, her son had grown sleepier and cried more frequently.

She wrote that she could not bear the thought that he might suffer because she had failed to protect him. She wrote that what she was about to do was "evil."

She then jumped out of her eighth-floor window. She left behind the note. She did not leave behind her son, strapping him to her body in a Ergobaby carrier, bringing him down with her as she crashed to the ground, the crack sounding like a gunshot to people passing by.

Ms. Wachenheim, 44, died. But her 10-month-old son, apparently cushioned by her body, survived. He bounced out of the carrier and suffered only a bruised cheek.

"I'm sure you understand, I'm absolutely overwhelmed with grief," her husband, Hal Bacharach, said in a brief telephone conversation Thursday from his apartment at the Sutton, a new sleek building at 147th Street and Bradhurst Avenue in Harlem, where he had lived with his wife.

"I have my son, who was lucky enough to survive, in my lap," Mr. Bacharach said, sounding in shock as he repeated similar words several times. "It's unbelievable. Right now my crying son is in my arms." A child could be heard whimpering as he spoke.

Ms. Wachenheim's leap was a jarring twist in the life of a highly educated, socially conscious woman who had been active in a women's group in her synagogue, B'nai Jeshurun on the Upper West Side, and, according to her college class notes, had been a coordinator for a Harlem tutoring program.

She was on leave from her $122,800-a-year job as an associate court attorney in the Manhattan State Supreme Court system, court officials said. She had worked for the courts since 1997, doing legal research and helping judges write opinions.

Christian Johnson, a lawyer who lives a few doors down, said he had seen Ms. Wachenheim twice last week. There was no indication "that anything was askew in their household," he said. "I was shocked."

The baby seemed normal, he said. Mr. Johnson would sometimes ride the train with Mr. Bacharach, who never said anything about developmental issues. "Hal never mentioned that to me," Mr. Johnson said.

But Mr. Bacharach's mother, Barbara Bacharach, said that her daughter-in-law had not been her usual self lately.

Mr. Johnson said he had overheard the couple arguing - which he said was very unlike them - about two hours before Ms. Wachenheim jumped. He paused in the hallway to make sure it was nothing serious, then moved on when it seemed like a normal marital spat. "He was just asking her why she didn't answer the phone and why wouldn't she pick up the phone," he said. "He just kept asking her and she wouldn't respond."

Several times in her note, according to the law enforcement official, Ms. Wachenheim expressed deep love for her son, referring to him as "beautiful."

She said that she would give her life to bring his health back and that she hated herself for the first time in her life. She believed that her son's falls might have brought about a serious medical condition, perhaps cerebral palsy or autism, which would have "lifelong consequences."

Her belief that she failed to prevent it caused her to "crumble." She wrote that she was depressed and could no longer socialize. She was sure that people would see her behavior as postpartum depression or psychosis.

Dr. Catherine Birndorf, a reproductive psychiatrist at Weill Cornell Medical College, said the word "evil" in the note stood out for her. "Usually these intensely lethal acts happen in the context of losing some kind of touch with reality," she said. "What mother in their right mind would kill their kid?"

Postpartum depression does not usually lead to suicide and homicide, she said, unless it is left untreated or progresses to more serious mental illness, like psychosis. She compared it to the case of Andrea Yates, the Texas woman who was found not guilty by reason of insanity of drowning her five children in the bathtub. Ms. Yates, who had been struggling with postpartum psychosis, thought that she was a bad mother and that she was protecting her children by killing them, Dr. Birndorf said.

About 10 to 20 percent of new mothers have postpartum depression, according to the state health department, and only 1 or 2 out of 1,000 new mothers have postpartum psychosis. Postpartum psychosis is characterized by delusions, often about the baby, agitation, anger, paranoia, and sometimes commands to harm the infant. It has a 5 percent suicide rate and a 4 percent infanticide rate, according to the health department.

Ms. Wachenheim was valedictorian at Colonie Central High School, near Albany, and graduated from what is now known as the University at Buffalo, and from Columbia University Law School. In 1993, she traveled to Pakistan to work in a law office specializing in women's rights and worked on subjects like "honor killings" of women suspected of adultery, according to an article at the time in The Times Union of Albany.

Mr. Bacharach said he met his wife on a bus to Boston and was smitten by her "innate kindness." They were married in 2009, two years after she bought her apartment at the Sutton for $190,750, according to city records.

It is across the street from Jackie Robinson Park, where neighbors said Ms. Wachenheim took her newborn to the outdoor pool in the summer.