HONOLULU -- The man who threw a toddler to his death onto the H-1 Freeway will be using the insanity defense, his lawyer told KITV.

Matthew Higa's attorney said his client's mental problems began before Higa started using drugs.

Despite the apparent madness of the killing, it is far from certain that Higa can avoid conviction with an insanity defense. The key question is whether mental illness caused his drug abuse or the other way around.

At Oahu Community Correctional Center on Monday morning Higa, 23, was brought from a special holding room to a closed circuit TV camera for his arraignment. He is being housed separately from other inmates for his own protection, according to authorities.

"He's doing the best he can under the circumstances," defense attorney Randy Oyama said.

Higa pleaded not guilty on Monday even though he apparently does not deny throwing 23-month-old Cyrus Belt off the Miller Street pedestrian overpass.

"There are definite issues about his state of mind at the time," Oyama said.

Over the next few months, Higa will be examined by a three-expert panel to see if he is mentally fit for trial and to give opinions about whether Higa should be held mentally responsible for the crime. If his mental illness is blamed on drugs, it will not protect him from criminal conviction.

"There are signs out there that clearly showed he was laboring under the influence of some kind of mental problems," Oyama said.

Oyama said he suspects Higa's mental problems began in grammar school when his mother died in a car accident. Then, four years ago, his care-giving grandmother died soon after a close friend was killed while speeding with Higa.

"It was his best friend. They used to hang out. They used to be real tight together, and I think he kind of withdrew from people," Oyama said.

Oyama said Cyrus' mother and her boyfriend's neglect and drug use could be relevant.

"It could be tragic on many different levels," Oyama said.

If Higa is acquitted by reason of insanity, that does not mean he would be free. Most murder defendants acquitted for mental illness spend many years, sometimes their entire lives, in the state hospital.