Pretoria, South Africa (CNN) -- Oscar Pistorius prosecutor Gerrie Nel wants the South African track star sent for independent psychiatric evaluation, he said Monday, in a move that could delay the athlete's murder trial for a month or more.
A psychiatrist testified Monday that Pistorius has an anxiety disorder stemming from his double amputation as an infant and his unstable parents.
He's depressed now and feeling guilt from having killed his girlfriend, Reeva Steenkamp, defense witness Dr. Meryl Vorster said on the stand.
Nel responded by comparing the athlete's mental state to post-traumatic stress disorder and saying the law required psychiatric observation.
The prosecutor's extremely unusual move is essentially an effort to maneuver the court into considering an insanity or "capacity" defense even though the athlete's legal team is not mounting one, CNN legal analyst Kelly Phelps said. Phelps, a criminologist and law lecturer at the University of Cape Town, said she had never seen this done before.
Pistorius' lead defense lawyer, Barry Roux, is fighting back, saying Nel is oversimplifying the law.
The question was left unresolved when court adjourned for the day. Nel is due to submit his application on Tuesday after both sides have finished questioning Vorster.
The psychiatrist took the stand Monday morning for the defense, going all the way back to when the disabled sprinter was 11 months old.
Pistorius would have experienced the amputation of both of his legs below the knee at that time as a "traumatic assault" because he was too young to speak or understand what was happening to him, she said in court.
His parents then put pressure on him to appear normal, and his mother abused alcohol at times after she and Pistorius' father divorced, she said.
She raised him and his siblings "to see their external environment as threatening" and "added to the anxiety," Vorster said.
Pistorius earlier testified that "Everything I learnt in life, I learned from her."
The defense is trying to show that Pistorius made a genuine and reasonable mistake and responded reasonably on the night he killed Steenkamp, 29, a model and law school graduate.
Pistorius, 27, admits that he fired four bullets through a closed door in his house, killing Steenkamp, but says he thought he was protecting himself from a burglar.
Whether the judge believes he acted reasonably could mean the difference between a verdict of culpable homicide, a lesser charge than murder, and an acquittal, according to Phelps.
If Pistorius is convicted of murder, he faces at least 15 years in prison, and possibly life. South Africa does not have the death penalty.
A verdict of culpable homicide would leave the sentence at the discretion of Judge Thokozile Masipa.
Vorster said the athlete's general anxiety disorder meant he felt that his safety was threatened even when objectively it was not, she said.
And she said he was "devastated that he killed his girlfriend."
Steenkamp's mother, who has been in court for most of the trial, was not there Monday to hear the evaluation.
Fight or flight
Vorster also addressed the question of why Pistorius took his gun and went toward the sound of what the thought was danger rather than trying to get away.
Because of his disability, she said, when he faces a fight-or-flight situation, he cannot flee, so his instinct is to fight.
Nel then pressed the psychiatrist on whether Pistorius was mentally ill and whether he could distinguish right from wrong.
She said he could.
Many people have general anxiety disorder, and it does not imply that one has lost touch with reality, she said.
The psychiatrist took the stand on day 30 of a high-profile trial that has gripped South Africa and much of the world.
She could be the last witness. Roux said last week that he expected to wrap up his case Tuesday. New psychiatric evaluation could push the end of the trial a month or more away.
When the defense is done presenting its evidence, both sides will make closing arguments, and then Masipa will retire to consider her verdict. South Africa does not have jury trials, but she is being assisted by two experts called assessors.
Nel has fought aggressively to show that Pistorius argued with Steenkamp before killing her.
The trial has seen Pistorius break down repeatedly, crying, wailing and sometimes throwing up as the court sees and hears evidence about Steenkamp's death.
Vortser said the athlete's physical distress was real.
He listened with his head lowered as the psychiatrist testified Monday.
Earlier evidence has included graphic photos of the wounds; testimony from neighbors, friends, police and pathologists; and the actual door through which Pistorius fired four hollow-tipped bullets on the fateful night.
Argument or error?
There is no dispute that Pistorius shot and killed Steenkamp in his home early on the morning of Valentine's Day 2013.
Nel tore into Pistorius over five days in court in April, saying the Paralympic medalist had argued with Steenkamp and killed her on purpose. He tried to force Pistorius to look at a picture of Steenkamp's head after the shooting, accused Pistorius of being selfish and possessive, and said he refused to take responsibility for his actions.
The defense team is seeking to cast doubt on the state's case and needs only to show there is a reasonable doubt that Pistorius meant to kill Steenkamp.
Masipa must decide whether Pistorius genuinely made a terrible mistake or whether he murdered Steenkamp intentionally.
Pistorius is arguably the world's most famous disabled athlete, known as the "Blade Runner" for the carbon-fiber blades he runs on.
He fought for -- and won -- the right to compete against able-bodied runners at the Olympics after his Paralympic success.
Sports fans worldwide saw Pistorius a symbol of triumph over physical adversity.