An affidavit by an investigator released on Friday by the York County Sheriff’s Office specifies that "numerous notebooks” containing “cryptic writing” were found in his residence.
"Numerous notebooks were located with cryptic writing with different designs and emblems" in the former NFL player’s home, the warrant reads.
The affidavit states that "detectives had also learned that Phillip Adams had been acting differently and possibly following a new religion or ideology.”
"Detectives were unclear if this was a potential motive or if there was another motive involved,” the document adds.
Authorities say Adams fatally shot a renowned doctor and five others at a house in the Rock Hill area on 7 April. Adams was later found dead by a self-inflicted gunshot wound.
The 32-year-old allegedly opened fire on Dr Robert Lesslie, 70, his wife, Barbara Lesslie, 69, and their grandchildren Adah Lesslie, nine, and Noah Lesslie, five.
James Lewis, 38, was found dead outside the home, police said he was working for a company focusing on air conditioning, roof, and heating repair.
A sixth victim was hospitalised with “serious gunshot wounds” following the incident but died of his injuries at a local hospital following the incident, reports said.
The warrants reveal that nine firearms were also found by officers at Adam’s home and three dozen casings for 45 caliber and 9mm ammunition, along with more than 20 "projectiles” were found at the Lesslie home.
Several mobile phones were also found at the Lesslie residence, according to the warrant, including an iPhone in a red case that had a phone number officers then tracked to Adams.
The former cornerback, who was born in Rock Hill, played for a number of teams including the San Francisco 49ers and the New York Jets during a six-year career.
The suspect’s sister, Lauren Adams, previously told USA Today that her brother had been exhibiting “unusual behaviour” ahead of the shooting.
“We definitely did notice signs of mental illness that was extremely concerning, that was not like we had ever seen,” she told the newspaper. “He wasn’t a monster... His mental health degraded fast and terribly bad”.
Alonzo Adams, the father of the suspect, previously confirmed that Dr Lesslie had been his son’s doctor at some point in the past.
A person briefed on the investigation who spoke to the Associated Press on the condition of anonymity because he wasn’t authorised to speak publicly has previously said that Robert Lesslie had treated Adams.
Additional reporting by the Associated Press