CENTENNIAL — At least one mental-health professional who recently examined James Holmes has made a diagnosis that supports an insanity plea for the Aurora movie theater shooting suspect, a lawyer for Holmes said in court Monday.

In making a formal request to change Holmes’ plea, attorney Dan King referenced just-concluded mental-health evaluations of Holmes that mean the defense can now “in good faith tender that Mr. Holmes was not guilty by reason of insanity.”

“We now have a diagnosis,” King said. “We now have an opinion by qualified professionals.”

The admission is the closest thing yet to a confession in the case. To support a successful insanity defense, a diagnosis would have to conclude that Holmes was so mentally ill at the time of the July 20 shootings — which killed 12 people and wounded 58 by gunfire — that Holmes’ judgment was affected and he could not distinguish right from wrong. As the Colorado Supreme Court has ruled, insanity pleas are “in the nature of confession and avoidance.”

Monday’s hearing was the first step in what will be a weeks-long process to formally change Holmes’ plea to insanity.

Because Holmes refused to enter a plea at his arraignment in March, his attorneys needed to show Monday that they have “good cause” now to swap in the insanity plea for the standard not guilty plea the judge earlier entered on Holmes’ behalf. King said Holmes’ attorneys have been “working on all cylinders” to investigate Holmes’ mental health and could only offer the insanity plea after receiving the professional opinions.

“We had various things our experts wanted to do,” King said.

Prosecutors objected to the delay the plea change will bring to the case. Senior deputy district attorney Jacob Edson said of the 28 victims his office was able to reach about the plea change, 19 were opposed to allowing it.

“Our largest frustration is the amount of time it’s taken to get to this point,” Edson said.

Judge Carlos Samour, though, ruled Holmes’ attorneys had shown “good cause” for the plea change. He cited the recently completed mental-health evaluations, along with Colorado Supreme Court precedent that judges should be permissive in plea-change requests that aren’t made close to trial.

“Based on the record made by Mr. King, good cause has been established,” Samour said.

That means Holmes remains on track to plead not guilty by reason of insanity, but the plea has not yet been entered for him.

Samour still must formally accept the plea. He can only do that after advising Holmes of the consequences of such a plea — including a court-ordered psychiatric examination — and Holmes has acknowledged the ramifications.

Holmes’ attorneys have objected to the advisement Samour intends to give Holmes. They contend, for instance, that telling Holmes he must cooperate with the examination violates his rights against self-incrimination.

Samour said Monday he will rule on those objections by the end of the month. He set a May 31 court date to give Holmes the advisement and officially accept — or, potentially, reject — the insanity plea.

Holmes is charged with 166 counts of murder, attempted murder and other offenses in connection with the attack at the Century Aurora 16 movie theater.

His trial is currently scheduled to begin in February 2014, but it could be moved back if the insanity plea is accepted and an independent psychiatric evaluation is ordered.

Prosecutors have said they will seek the death penalty in the case.

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold