He then allegedly turned his attention to Keith's wife, Jennifer, and their children, Sophia, 8, and Jesse, 11, stabbing them repeatedly.
The charges did not disclose a motive, and a spokesman for the Monmouth County prosecutor said there would be no comment until an 11 a.m. news conference.
All four members of Keith Caneiro's family were "victims of homicidal violence", the prosecutor said, adding there was no indication of suicide.
That fire was a ruse to create "an illusion" that both families were targeted and to destroy evidence from the Colts Neck slayings, Gramiccioni said.
Paul Caneiro was previously arrested and charged after he set fire to his own home in Ocean Township with his wife and two daughters still inside.
Caniero, 51, has been in custody since authorities accused him of setting fire to his own home.
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He has not entered a plea on the new charges, the records show.
The details of the heinous crime spree came just hours after Monmouth County prosecutors filed murder charges against Paul Caneiro in the November 20 eruption of violence in Colts Neck and Ocean Township, New Jersey.
Honecker has not commented on the reports on the new charges.
Keith Caneiro was shot and found dead in front of the large 5,700-square-foot home, and the mother and two children were found inside, according to Monmouth County Prosecutor Christopher Gramiccioni. No one was injured in that blaze, which broke out hours before the one in Colts Neck.
The charges against Caneiro note only that the offenses were committed on November 20.
Attorney Robert Honecker had previously maintained his client was innocent and said Caneiro was "devastated by the news of his brother and his family's deaths". "He fully expects to be vindicated when this case finally resolves".
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Grammiccioni said he was omitting a lot of details because of the pending case, but this is his rough description of the murders.
Paul and Keith Canario were partners in a technology consulting firm and a pest control business in Asbury Park.
Paul Caneiro was charged last week with aggravated arson stemming from the fire at his house.
Paul and Keith Caneiro are listed as the sole officers of a company that involved the sale and service of computers, New Jersey business records show. A 2007 filing indicates that Keith was the company's president.
Clarification: This article has been updated to clarify that Jennifer Caneiro was stabbed and shot, according to the Monmouth County prosecutor.
The brothers were best friends who talked nearly daily, Honecker told CNN, adding he had no reason to believe there was animosity between them.
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