(CNN) A man killed in a car explosion over the weekend in Allentown, Pennsylvania, intentionally carried out the blast that also killed his 2-year-old son and another man, authorities said Thursday.

Jacob Schmoyer, 26, revealed his intentions in four letters sent to his family and law enforcement, said Don Robinson, special agent in charge of the investigation for the Bureau of Alcohol, Tobacco, Firearms and Explosives.

"All the letters indicated that Mr. Schmoyer was to blame. He took blame for it."

The explosion occurred around 9:30 p.m. Saturday in Allentown, about 60 miles north of Philadelphia, killing Schmoyer, his son, Jonathan, and David Hallman, 66, an associate of Schmoyer's, Robinson said.

Two letters "indicated that (Schmoyer) intended to target Mr. Hallman, and in one of the other letters we were able to confirm that he intended to take his son with him also."

Read More