The deadly car explosion in Allentown, Pennsylvania, on Saturday that killed three people, including a toddler, was a targeted murder-suicide by a depressed father, authorities said early Thursday.

Jacob Schmoyer killed his 2-year-old son, Jonathan, and friend David Halman, 66, when he used a homemade explosive device inside his car on Saturday. A spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives said in a press conference that Schmoyer sent letters to the Allentown Police Department outlining his plans to kill both Halman and his son.

"He was miserable. Basically, the four letters describe a miserable life, he was unhappy himself," Don Robinson, the special agent in charge at the ATF Philadelphia field division told reporters Thursday. "I don't know if shame is too strong of a word because of what he did later, but he admitted to a lot of criminal acts, he didn't think it was going to get any better. There was a lot of hatred there, and obviously, some directed at Mr. Halman and his son."

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Jacob Schmoyer and his son, Jonathan. via Facebook

The Allentown Police Department first received a letter from Schmoyer, 26, on Tuesday, after the explosion and then received another three the following day. Robinson told reporters Thursday Schmoyer's letters outlined the materials and construction of the device he intended to use as well as his decision to kill Halman and his son. Schmoyer's letters were dated on September 29, the day of the explosion and detailed other crimes he committed in the past, such as petty theft and burglaries.

Authorities are still working to confirm the authenticity of the letters but are confident they are all from Schmoyer. Robinson did not describe the relationship between Schmoyer and Halman, but did say the pair knew each other and were in contact in the hour leading up to the explosion.

Family members told NBC News they informed federal officials that Schmoyer had a history of schizophrenia and had previously received treatment.

Robinson initially said at the press conference Thursday he was unaware of a history of any mental illness, but later told NBC News that the ATF had only been told Schmoyer had a history with depression.

Schmoyer's father, Glenn, said in a statement to NBC News earlier this week that he was "traumatized" by what happened and that he was extremely close to Jacob.

"He always played with his son and I just have so many good memories of them two together," Glenn Schmoyer's statement said. "My son was a very good man and he would give the shirt off his back to help someone in need. He was never selfish. Things meant nothing to him, people did."

Authorities determined all three victims were inside the car at the time of the explosion. Halman was seated in the passenger seat and Schmoyer's toddler son was in a car seat behind him on the driver's side of the vehicle.

"We're pretty confident that the device was placed either on the center console or near the center console front passenger seat," Robinson told reporters Thursday. Authorities did not say where Schmoyer constructed the device but were certain there are no more devices based on the description in the letter, the crime scene, and in searches since the explosion.