A Massachusetts man accused of killing his wife and two young children allegedly said in a note that hatred for his in-laws fueled his rage, MyFoxBoston.com reports.

Prosecutors say Thomas Mortimer IV murdered his wife, mother-in-law, and two children -- ages 4 and 2 -- after his wife confronted him about a $2,499 bounced check to the IRS.

Authorities discovered the bodies of Laura Stone-Mortimer, Ragna Ellen Stone, Thomas "Finn" Mortimer and Charlotte Mortimer on June 16 in the couple's home in Winchester, Mass.

Mortimer's wife, Laura, and mother-in-law died from stab wounds and his children's throats were cut.

When police later searched the home, they uncovered a note with a chilling confession allegedly written by Mortimer about the killings.

Mortimer said in the type-written letter that he should have written a book about dealing with his wife's dysfunctional family and not let his anger fester "until one murderous night," according to MyFoxBoston.com.

"I am ashamed, frightened, relieved, surprised that I murdered my family, disgusted with myself," Mortimer wrote in the purported confession.

The letter was included in a nine-page summary of the case filed by prosecutors and made public yesterday at the request of news outlets.

Mortimer, a 43-year-old sales executive, pleaded not guilty on Sept. 2 to four counts of first-degree murder.

His lawyer, Denise Regan, has said he has mental health issues. Regan had argued that the nine-page statement was "excessive," and could affect Mortimer's right to a fair trial if the details were released to the public.

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The Associated Press contributed to this report