A forensic psychiatrist has testified that Lou Gehrig's disease is the reason a former sheriff's deputy in Wisconsin killed his wife and sister-in-law, whom the defendant described in a suicide note as his threesome partners. Andrew Steele, 40, has pleaded not guilty by reason of mental disease in the shooting deaths last August of his wife, 39-year-old Ashlee Steele, and her 38-year-old sister, Kacee Tollefsbol, of Lake Elmo, Minnesota. Taking the stand for the defense on Day 2 of the insanity phase of his trial  Dr. Doug Tucker testified that a rambling note Steele wrote about suicide and sexual relations with his wife and sister-in-law is delusional and shows Steele's brain was deteriorating because of ALS. Tucker said that was the reason Steele killed the two and that he was 'unable to conform his conduct to the law.' Scroll down for video . Back in court: On Day of his insanity trial, former Wisconsin sheriff's deputy Andrew Steele, 40, was wheeled into the courtroom to hear testimony from a forensic psychiatrist and his sister-in-law . Tragic sisters: This undated photo provided by Rachelle Putnam shows Ashlee Steele, left, and her sister, Kacee Tollefsbol, both were found dead of gunshot wounds at Steele's home in Fitchburg, Wisconsin . Steele, a former Dane County deputy, was diagnosed with the progressive neurodegenerative disease last June. The defendant claims he blacked out during the killings. And on Tuesday, the defense kicked things off detailing Steele's condition and how science will prove him not responsible. The prosecution disagrees. The State asked all witnesses if Steele had memory issues before that day in August, and they all said no. In a mostly incoherent, stream-of-consciousness letter written by Steele the day before the murders, the former deputy claimed he had had a sexual relationship with his sister-in-law and enjoyed 'numerous threesomes' with Kacee and his wife, reported the station WIBA. The typo-riddled note also stated that when Kacee and Ashlee learned of his ALS diagnosis, they proposed a suicide pact. According to Steele, the women wanted to jump off a cruise ship together, poison themselves with carbon monoxide inside a car, or shoot themselves with his service weapon. Video from WMTV . ALS: Andrew Steele, pictured in court in 2014, penned a rambling note a day before the killing claiming that he had had threesomes with his wife and sister-in-law, and that the three of them had made a suicide pact . Case: Steele argued last month that he was not guilty by reason of mental disease or defect to first-degree intentional homicide charges because he could not resist impulses . Tearful testimony: The slain sisters’ third sibling, Hailee Mesiterling, said from the stand that Andrew Steele was in love with Tollefsbol . 'See you all on the other side,' the note concluded. The missive was last edited on Steele’s phone just after 6am August 22. About seven hours later, a mortally wounded Kacee Tollefsbol called 911 saying she and her sisters had been shot,. Dr Tucker testified that there is no evidence to support Steele's claims that he had been in a three-way relationship with his wife and sister-in-law. Friends of Kacee Tollefsbol also said in court that her and Steele's relationship did not go beyond that of a brother and sister, reported Wisconsin State Journal. However, the slain sisters’ surviving sibling, HaileeMesiterling, said from the stand that Andrew Steele was in love with Tollefsbol and acted inappropriately towards her during a family vacation last summer. In her opening statement Tuesday, Jessa Nicholson, Steele’s defense attorney, said her client maintains that on the day of the killings he and Ashlee had sex involving bondage, and a zip tie accidentally fastened too tight around her neck, according to WKOW. Death: Sisters Ashlee Steele, left, and Kacee M. Tollefsbol, right, were shot dead leaving six children behind between them . In happier times: Ashlee Steele, left, had been working tirelessly to raise money to pay for her husband's treatment . Steele’s last memory from that day, according to his attorney, is of him trying to find a pair of scissors to cut the restraint around his wife’s neck. He then blacked out and awakened later in a hospital bed with a tube down his throat. Unable to speak, the 40-year-old wrote notes asking if he had been in a car accident and inquiring after his wife. The trial is expected to continue into next week. If the jury finds him responsible for the double-homicide, Steel could face up to a life sentence without parole. Ashlee Steele, 39, was found shot in the head in the master bedroom and cops found Andrew Steele sitting in a laundry room with a pistol. The former deputy had started the dryer and vented it back into the room and he had set up a grill loaded with hot charcoal in the room in a possible suicide attempt by carbon monoxide. Accused: Andrew Steele is wheeled out of his home in a stretcher Friday after allegedly murdering his wife and her sister . The killings came just days after the victims and the suspect's friends took part in the ALS ice bucket challenge, raising $23,000 towards the ailing deputy's medical expenses. In his letter, Steele lamented that collected funds were not enough to cover their family's expenses.  . Between the two of them, the slain sisters have left six children behind. As he addressed the court, Steele's speech sounded slurred as he waived his right to review the statements of potential jurors claiming to be unable to serve during his three-week trial. Around 15 jurors will be picked from 120 potential jurors for the trial which is scheduled to begin with jury selection on April 13. It will be a two-phase trial, the first to establish guilt, the second to decide whether he was legally responsible for his actions. The dramatic case saw Steele wheeled into the courtroom by bailiffs at the start of a status conference before Circuit Judge Nicholas McNamara. Challenge: His wife, Ashlee Steele spearheaded the effort to raise money for her husband dubbed 'Tough as Steele', posting videos of the ice bucket challenges on her Facebook account . Family: The couple, pictured, had two children, and had been together for 26 years . His lawyer, Jessa Nicholson, said that Steele can still walk and that his speech has been affected by his ALS, but that otherwise he was doing well. ALS does not affect the brains of most people who are diagnosed with the disease, but Nicholson said about 20 per cent experience dementia. Nicholson wrote that Steele has suffered damage to his hippocampus, the emotional center of the brain, causing poor impulse control and loss of memory. His frontal lobe was also damaged, which can cause loss of inhibition, personality changes and loss of the ability to conform his conduct to the law.
Andrew Steele, 40, claims that the terminal disease affected his brain .
Steele is charged with first-degree intentional homicide for the murders of his wife, Ashlee Steele, 39, and her sister, Kacee Tollefsbol, 38, in 2014 .
He has plead not guilty by reason of mental disease or defect .
Jury was presented with Steele's letter written before slayings claiming he had had threesomes with Ashlee and Kacee .
His attorney said Steele's last memory from day of killings was looking for scissors to cut zip tie from around his wife's neck during bondage sex .