Castillo's attorneys are trying to prevent his two statements to police from being admitted as evidence at trial. Prosecutors played a tape of the first of those statements for the judge Wednesday to verify its authenticity. The jury has not yet been chosen.
Asked by homicide Detective Raymond Yost why he killed his children, Castillo let out a long sigh before describing a custody battle with his former wife.
"We've been going through an extremely hard divorce. ... My wife had denied my visitation," Castillo said. "I felt like I didn't want them to be in this world anymore."
Castillo described how he took his children to the Maryland Science Center in Baltimore and then checked into a Marriott hotel downtown.
"My plan was, after we had a good day, to take their lives," Castillo said.
Castillo ordered room service and distracted his sons with video games as he took Athena into the bathroom.
"I began her bath, and I held her underwater for, I don't know, 10 minutes," he said. "I dried her off and put her in bed."
He described repeating the process with the two boys. The order in which Austin and Anthony were killed was not clear from his statement.
Castillo then described how he stabbed himself in the neck in a bid to "cut a major artery and bleed to death." He took some pain pills, passed out and awoke the next morning.
"I felt unfortunate because I woke up," he said.
Castillo said he called the hotel's front desk and told the clerk, "I took the children's life last night. I tried to take mine without success. Please call the cops and have them do what they need to do."
Castillo told the detective that he deserved to die for his crimes.
Jury selection for Castillo's trial will begin after Baltimore Circuit Judge Wanda K. Heard rules on whether the statements can be admitted as evidence. Castillo faces life without parole if convicted.
Castillo has behaved erratically in court in the past but was calm as the tape was played. He has a history of mental illness, including a diagnosis of mood disorder and narcissistic personality disorder, according to court records.
His ex-wife, Dr. Amy Castillo, a pediatrician, has attended portions of the proceedings but was not in court when the tape was played.