The fate of 19-year-old Nikolas Cruz, who faces 17 counts of first-degree murder in the shooting at Marjory Stoneman Douglas High, will depend on his mental state and the wishes of the victims' families, which have a say in how the prosecution proceeds.

Broward County Public Defender Howard Finkelstein, whose office is representing Cruz, said there were so many warning signs that Cruz was mentally unstable and potentially violent that the death penalty might be going too far. Finkelstein said Cruz would likely plead guilty if prosecutors opt not to seek the death penalty.

"Because that's what this case is about. Not, did he do it? Not, should he go free? Should he live or should he die," Finkelstein said. "He will never see the light of day again, nor should he. But I know personally I am very upset and angry that we all failed to spot a problem and do anything as a result."

Michael J Satz, the state attorney for Broward County, said in an email that "This certainly is the type of case the death penalty was designed for." He called the slayings "absolutely horrific and tragic." However, he also said his office is working with law enforcement and will announce later what penalty it plans to seek.

The prosecution will likely take years.

A major issue for the courts will be Cruz's mental state. Officials have said he underwent unspecified treatment at a mental facility but quit after his mother died in November. His father had died some years earlier. Without any living parents, he was taken in by a local family.

Cruz's attorney, Assistant Public Defender Melisa McNeill, told reporters after Cruz's initial court appearance that he had become unmoored from society and had no support network to lean on.

"When your brain is not fully developed, you don't know how to deal with these things," she said. "When you have the lack of impulse control that a 19-year-old has, that affects the behaviour you exhibit."

McNeill also said of Cruz: "He's sad, he's mournful, he's remorseful. He's just a broken human being."

An initial decision will be whether Cruz is mentally competent to understand legal proceedings and assist in his own defence. Experts say it's a relatively high bar to clear to be declared incompetent and McNeill said Cruz is "fully aware of what is going on."

Cruz could try to plead innocent by reason of insanity, which also rarely works. James Holmes, the shooter who killed 12 people and wounded 70 in a Colorado movie theatre in 2012, was convicted despite pleading insanity and was sentenced to life behind bars.

Australian Associated Press