Ummad Rushdi, 30, told Jawwad Rushdi, 32, on Monday that Hamza Ali's killing occurred in a suburban Philadelphia home a day earlier, the warrant said.
Ummad Rushdi told Jawwad Rushdi that he shook the crying baby and that the infant then became unconscious, according to the affidavit.
"Jawwad stated that Ummad told him that he attempted CPR, but was unable to revive Hamza," police wrote in the affidavit. "Ummad further advised Jawwad that he took the child from the residence and buried him, but did not tell him where."
The warrant sought the child, blood or DNA evidence and any digging tools from Ummad Rushdi's home in a suburban development outside York. A search dog was used to comb areas outside the home, including a soybean field beyond the backyard.
"Realistically, the baby could be anywhere between here and Delaware County," said York Area Regional Police Sgt. Jeffry Dunbar.
Police said Ummad Rushdi is the boyfriend of Hamza's 20-year-old mother, Zainab Gaal. The couple had been together about three weeks, police have said.
Gaal told police that Ummad Rushdi and her son left the home in Upper Darby in Delaware County early Sunday, and that he did not have permission to take the boy.
Police were at the York home searching Thursday, one day after they spent several hours fruitlessly looking for any sign of the child in a wooded area along the Susquehanna River in Columbia, east of York. That is near where Rushdi's car had apparently become disabled.
Ummad Rushdi was arrested Wednesday morning and jailed on $750,000 bail after being arraigned in Upper Darby on charges of kidnapping, unlawful restraint, false imprisonment, child custody interference and endangering a child's welfare. Court officials said Thursday that Rushdi was not yet represented by a lawyer.
Ummad Rushdi's roommate said in an interview inside the home Thursday that they spoke on Monday, and it appeared to him that Ummad Rushdi was packing up a vehicle and possibly moving.
"His face was completely white, like he was spooked or something," Darryl Heverling said. Heverling offered him some leftover hamburger casserole, but he turned it down.
"It was really disturbing," Heverling said. "He said something about meditating and people with guilty consciences and clearing his mind."
Heverling said Ummad and Jawwad are brothers, and that Gaal, Hamza and Ummad Rushdi all lived in the house. He was unsure if Gaal and Rushdi had a job.