The Virginia lawyer accused of invading a McLean couple’s home by using a Taser, holding them hostage for three hours, then slashing one victim’s throat and repeatedly stabbing the other plans to use the defense of “involuntary intoxication” at his trial, starting Monday in Fairfax County.

Andrew Schmuhl of Springfield, a 32-year-old former judge advocate and military magistrate in the Army, attacked the couple because the man had recently fired Schmuhl’s wife from an Arlington law firm, Fairfax prosecutors said. The wife, Alecia Schmuhl, 31, waited outside and drove the rented getaway car after the incident, which ended when they were pulled over on the Capital Beltway and Andrew Schmuhl was found to be wearing only a diaper, according to police and prosecutors.

The Schmuhls will be tried separately, with Andrew Schmuhl going first. His lawyers have said that he suffered a spinal cord injury in a 2012 accident and court records show he had been receiving disability payments and not working since then. The defense’s pleadings state that they want a mental health expert to testify that the effects of various medications Schmuhl was taking caused him to be so intoxicated he was unaware of what he was doing during the three-hour assault on the McLean couple.

The involuntary intoxication defense is not new, is rarely successful, and Virginia even has a specific jury instruction for judges to give when defendants make that claim, often in drunken driving cases.

A number of Virginia lawyers said they were unfamiliar with anyone who had won an acquittal in the state, even in a drunken driving case, but there have been some effective involuntary intoxication claims elsewhere. Last year in St. Paul, Minn., a woman charged with trying to kill and assault her two small children was released when prosecutors decided that the charges could not stand “in light of the defendant’s involuntary intoxication at the time of the charged incident.” A Columbia, Mo., woman who was convicted of causing a fatal wreck while driving the wrong way on Interstate 70 has been granted a new trial because she may have been secretly given a “date rape” drug before taking the wheel. And in Colorado, authorities are anticipating involuntary intoxication cases where defendants claim they unknowingly ate high doses of edible marijuana.

Experts said using such a defense for an extended set of actions such as Schmuhl’s may be more problematic. “The idea that you could concoct controlled substances into a cocktail,” said Joshua Marquis, an Oregon county prosecutor and spokesman for the National District Attorneys’ Association, “that they then do something they wouldn’t ordinarily do, such as entering someone else’s home and harming someone, and they would be unaware the actions they were doing is criminal, seems wholly unbelievable to me.”

Marquis noted that “the law does not punish rational versus irrational conduct. If we [prosecutors] had to explain why someone committed murder or rape, we would lose a lot of cases. Motive is something the prosecutor never has to prove.”

Veteran Leesburg defense lawyer Alex Levay said, “It’s a difficult defense, to say the least. It is a form of temporary insanity. But not only do you have to be intoxicated unwillingly and unknowingly, you have to be so unsettled you can’t distinguish between right and wrong.” Though he was not closely familiar with the specifics of the Schmuhl case, he said the defense’s “bigger problem is how did all these different things happen if you were under the influence? It does take a level of sophistication to commit certain crimes, so that intoxication is a difficult defense.”

Bradley R. Haywood, Schmuhl’s lead attorney, declined to comment on what exactly caused his client’s intoxication. But court records show he has enlisted psychiatrist Eileen Ryan, of the University of Virginia’s Institute of Law, Psychiatry, & Public Policy, to testify as an expert regarding “the effects of intoxicants on a defendant’s mental state.” Haywood argued in one brief that “involuntary intoxication employs the exact same legal standard as insanity,” and that “just as a jury would need to hear from a psychologist to understand the effects of schizophrenia on a defendant … it would need to hear from a mental health expert in order to understand whether a defendant satisfies [the insanity standard] because of medication.”

Casey M. Lingan, the chief deputy commonwealth’s attorney of Fairfax, also declined to comment. But in his response to Haywood, he wrote that “expert testimony about a defendant’s mental health is not relevant to his guilt or innocence and is inadmissible.” He cited a 2009 Virginia Supreme Court case where a defendant was convicted, and appeals rejected, after taking a quadruple dose of Ambien along with Benadryl and a painkiller. The trial court ruled the defendant “knew or should have known he should not take the drugs in the quantity and combination that he did.”

Fairfax Circuit Court Judge Randy I. Bellows has not yet ruled on whether Schmuhl can use expert testimony to build an intoxication defense.

Schmuhl’s case allegedly was ignited when his wife, an intellectual property lawyer, was fired from the firm of Bean, Kinney and Korman on Oct. 29, 2014. Eleven days later, on Nov. 9, a man appeared at the McLean home of Leo Fisher, the managing partner of Bean, Kinney. Police said he claimed to be with law enforcement, then used a Taser on Fisher to incapacitate him. The suspect then used plastic ties to bind the hands of both Fisher and his wife, Susan Duncan.

What happened next might actually buttress the argument that Schmuhl was temporarily out of his mind. Prosecutors allege that Schmuhl spent three hours interrogating the couple, forcing Fisher to access his email account, and making bizarre accusations about a Mexican drug cartel, Fisher’s supposed role in a murder for hire, the Knights Templar and a sniper possibly targeting him. Police have phone records showing that Schmuhl made repeated calls to his wife waiting outside the home, both using “burner” cellphones, though her lawyers have claimed she was unaware of what was happening inside. Alecia Schmuhl will claim in her trial that she was manipulated and abused by her husband, her defense has stated, and she reportedly gave police a statement blaming her husband for planning the attack. Fairfax Commonwealth’s Attorney Raymond F. Morrogh rejected those claims, saying surveillance video showed her buying a Taser shortly before the attack, which he called “a torture session.”

Police later found traces of gasoline spilled on Fisher’s rug and in a water bottle in the Schmuhls’ rented SUV, along with an improvised electrical device, and prosecutors plan to allege that Schmuhl also intended to burn Fisher’s house down, court records show.

After Fisher and Duncan had been tied up and questioned for three hours, Schmuhl suddenly “jumped on top of me,” Fisher testified in a preliminary hearing last year. “He put a pillow on my head and slit my throat.”

Duncan said she ran into the room and Schmuhl fired a gun at her. “I felt the bullet, and I fell,” Duncan testified at the hearing, describing how the shot grazed her scalp before lodging in the ceiling. She said the man then began repeatedly stabbing her in the upper body until she played dead.

The Schmuhls fled and were located and stopped 30 minutes later on the Capital Beltway. Officer Daniel Custer testified that “Mr. Schmuhl exited the passenger seat wearing nothing but what appeared to be an adult diaper.” No one in the case has clarified why he would be wearing a diaper.

The Virginia model jury instruction for involuntary intoxication informs jurors that “Intoxication is involuntary when it is produced in a person without his willing and knowing use of” either liquor, drugs or another substance. “If you believe that the defendant, 1) was willingly and unknowingly made intoxicated by,” and here the judge is to insert either “the fraudulent contrivance of others; an accident or mistake; or an error of his physician,” and “The intoxication so unsettled the defendant’s reason that he did not understand the nature, character and consequences of his act, or he was unable to distinguish right from wrong; then

“You shall find the defendant not guilty.”

Ronald Bacigal, a University of Richmond law professor whose book “Criminal Offenses and Defenses in Virginia” is cited as a source for the jury instruction, said Schmuhl “has got a potential defense. It doesn’t come up a whole lot. People are skeptical.” He said one of the difficulties in using it as a defense is “you can’t say it lowered my inhibitions. You have to say the intoxication was so bad I couldn’t control my behavior.” He noted there was no “diminished capacity” defense in Virginia. “You’re either insane or sane.”

The lawyer for Fisher and Duncan declined to comment. Both testified to their ordeal at a preliminary hearing last year and are expected to testify in Schmuhl’s trial, which the lawyers have said may last three weeks.

Marquis, of the district attorneys’ association, said that Virginia’s jury instruction was unusual among state courts, but Minnesota has one very similar to it. And defense attorney Mark Todd of St. Paul used it for the first time in his 34-year career last year.

Jozetta R. Byrd was arrested in February 2015 and charged with attempted murder and first-degree assault for sitting on and choking her 8-year-old daughter until she passed out and then trying to kill her 3-year-old son. Todd learned that Byrd had been diagnosed with bipolar disorder and also was taking Prednisone for asthma, although it contains a steroid that a doctor said sparked a psychotic episode. Byrd had not previously been violent or in trouble with the law or had any domestic problems with her kids, Todd said.

“We had to prove that the psychotic episode,” Todd said, “was unexpected intoxication and caused by a medically prescribed drug, and that she was so mentally deficient that she did not understand the nature of the act.” He presented a doctor’s report and Byrd’s personal circumstances to Ramsey County Attorney John J. Choi, who dismissed all charges against Byrd in June 2015.

Also last year in West Palm Beach, Fla., a man who pleaded guilty to killing his wife and twin 7-year-old sons was granted a new hearing because he said his attorney wouldn’t let him use the involuntary intoxication defense. The man, Neal Jacobson, argued that he was taking two anti-depressants while suffering from hypothyroidism, and the combination caused him to go temporarily insane. An appeals court ruled that Jacobson’s lawyer was ineffective for telling him he couldn’t argue involuntary intoxication.