Wayne Treacy, the Pompano Beach teenager charged with attempted first-degree murder for brutally beating and nearly killing a Deerfield Beach Middle School student in March 2010, began his insanity defense in Broward Criminal Court on Wednesday, with defense attorneys calling to the witness stand a forensic psychologist who testified that the boy suffered from post-traumatic stress disorder and other psychological problems.

But a state prosecutor aggressively cross-examined the psychologist, forcing him to admit that he relied on the diagnoses of others in a 28-page report on Treacy’s mental health, and challenging assertions that the boy was legally insane when he attacked Josie Lou Ratley by shoving her to the ground at a school bus stop, and repeatedly slamming her head against the concrete, and then kicking her in the head while wearing steel-toed boots.

“As Wayne Treacy sits here today, do you consider him legally insane?’’ Maria Schneider, the assistant state attorney prosecuting the case, asked the psychologist on the witness stand, Dr. Phil Heller.

“No,’’ Heller said.

Heller, a clinical and forensic psychologist, said he interviewed Treacy at least nine times between April and October 2011, and found the Pompano Beach teenager to be suffering from post-traumatic stress disorder and other psychological problems.

Heller said Treacy was a bright boy with no violent past who excelled at school and faced a promising future until the day he witnessed the suicide of his older brother, Michael Bell, in October 2009.

Bell hung himself with an electrical chord from a tree in the parking lot of the New Covenant Church in Pompano Beach on a Saturday morning — a grisly scene happened upon by Treacy, who was on his way to the beach to celebrate his recent 15th birthday, testified the boy’s mother, Donna Powers.

Heller said Bell was “the father figure that Wayne needed.’’

“Michael Bell was his coach,’’ he said. “He would believe in him.’’

After witnessing his brother hanging from a tree, Heller said, something deep within Treacy changed.

Treacy began to experience classic symptoms of post-traumatic stress disorder or PTSD, Heller said, including nightmares, recurrent flashbacks, ranting tantrums, lack of sleep and skittishness.

He said Treacy had no memory of his attack on Ratley, either, and called that a clear sign of amnesia — another symptom of PTSD.

Heller added that Treacy’s PTSD manifested itself in other psychological disorders, including “dissociation” and “depersonalization, where you feel that you’re watching your life on TV, or you feel like you’re invisible. You’re just not part of life.’’

The trigger that set off Treacy on a violent rampage, Heller said, was Ratley’s text message to Treacy telling him to “go visit your dead brother.’’

“When he saw that text message,’’ Heller said, “...that horrific trauma that he saw five months before had now come back.’’

Yet, according to a log of messages exchanged between the two in the hours leading up to the attack, Treacy had threatened Ratley at least twice before she sent the text message about his brother.

But Heller insisted that Ratley’s text message about Treacy’s dead brother launched the boy into a “dissociative’’ episode, almost as if he were in a trance.