Cleveland Police Department A thank-you note to Cleveland police from Michelle Knight, one of the three women held for years in the home of Ariel Castro.

Declaring, "Life is tough, but I'm tougher!" Cleveland kidnap victim Michelle Knight thanked police in a handwritten note released Wednesday as prosecutors revealed that Ariel Castro's three victims kept diaries detailing the horrifying conditions they endured.

"I am overwhelmed by the amount of thoughts, love & prayers expressed by complete strangers," says the note, which was posted on a Cleveland police Facebook page and which the department confirmed as authentic to NBC News on Wednesday. "It is comforting.

"Life is tough, but I'm tougher!" Knight wrote.

Hennes Paynter via AP In a handwritten note released Wednesday, Michelle Knight, one of the three women held for years in the home of Ariel Castro, thanked Cleveland police and wrote, "Life is tough, but I'm tougher!"

Authorities disclosed Wednesday that Knight and the two other women who broke free from Castro's home in May — Gina DeJesus and Amanda Berry — documented their decade of captivity in a series of diaries.

In a sentencing memo filed ahead of Castro's formal sentencing hearing Thursday, Cuyahoga County Prosecutor Timothy McGinty wrote that a "remorseless" Castro "serially abused them physically, emotionally, and sexually on a daily basis."

"The entries speak of forced sexual conduct, of being locked in a dark room, of anticipating the next session of abuse, of the dreams of someday escaping and being reunited with family, of being chained to a wall, of being held like a prisoner of war, of missing the lives they once enjoyed, of emotional abuse, of his threats to kill, of being treated like an animal, of continuous abuse, and of desiring freedom," the memo says.

Castro pleaded guilty last week to 937 counts of rape, kidnapping and aggravated murder and was sentenced to life in prison without parole — plus 1,000 years. Prosecutors dropped 40 extra counts that were considered redundant.

The women were abducted between 2002 and 2004. They escaped in May after Berry broke through a storm door and screamed for help while Castro was out of the house.

While the horrors the women endured have been widely reported, the sentencing memo adds new details that demonstrate how Castro lured them to his house and then managed to keep them under his power for the better part of a decade.

The document isn't being published on NBCNews.com because it includes the name of Berry's daughter, now 6, who was born in captivity.

According to the memo:

Castro lured Knight into his car in 2002 by promising her he would give her a puppy for her son. He spotted Berry walking home from work in 2003 and asked whether she would like to visit with his daughter. And in 2004, he offered DeJesus a ride home from school.

Once in the home on Seymour Avenue, the women were restrained with chains, fed one meal a day and allowed one or two cold showers a week. Each of the women was sexually abused on a regularly basis.

Whenever one of them tried to escape, Castro would assault her and force the other two to watch.

In 2006, one of those assaults resulted in Berry's becoming pregnant. When the baby was born, she wasn't breathing, and Knight saved the child's live by performing CPR on her.

A few months later, Castro impregnated Knight. Castro beat and starved Knight in an attempt to abort the pregnancy. That attempt was successful, and it was the basis of the aggravated murder charge to which Castro pleaded guilty.

The memo also includes a statement from Dr. Frank Ochberg, a widely known clinical psychologist who wrote the first standard textbook on post-traumatic stress disorder.

Ochberg concluded that the women were so badly mistreated and manipulated that they developed "Stockholm syndrome" — slowly "bonding" with their captor in gratitude for rare small favors he would grant them.

"Little by little, you are allowed 'the gifts of life,'" Ochberg wrote. "You are like an infant, totally dependent on your mother for survival. As you receive these gifts of life, without consciously realizing what is occurring, you feel some warmth — even love — toward that life giver."

"But I know this," he wrote. "They came out alive. They came out when they could come out. They acted with fortitude, courage and grace. We have reason to be proud of them and to be inspired by them."

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