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Sentencing is underway for Ariel Castro.
An FBI agent says a letter written by Ariel Castro contained an attempt to explain his actions in the imprisonment of three women for about a decade.
Special Agent Andrew Burke testified at Castro's sentencing Thursday in Cleveland. Burke says the note included Castro's comment saying, "I am a sexual predator."
Burke also testified that Castro would occasionally pay his victims after raping them. But he then would require them to pay him if they wanted something special from the store.
The agent also described some of the conditions found inside the Cleveland house.
Burke said bedroom windows were boarded shut from the inside with heavy closet doors. Burke says a door to one bedroom was modified with locks on the outside and the doorknob removed.
He says the house was divided in ways to make it more secure and to hide the existence of rooms.
Burke says alarm clocks were wired to the front door, and some rooms had ventilation only to the attic because of the boarded-up windows. There also were chains in the women's rooms.
Cuyahoga County Sheriff's Detective Dave Jacobs says that in an interview, Castro said: "I knew what I did was wrong."
Castro pleaded guilty last week to 937 counts including aggravated murder, kidnapping, rape and assault in a deal that could bring a life sentence plus 1,000 years.