The husband of a woman murdered in Belfast last year has filed a wrongful death suit against Todd Gilday, the man convicted of shooting her and seriously injuring her son with a shotgun blast.

The suit filed in Waldo County Superior last week reveals publicly for the first time that Gilday apparently shot Lynn Arsenault unintentionally when he angrily burst into the house she owned on Aug. 28, 2013, looking for her son.

The timing of the suit by Arsenault’s husband, Donald Arsenault Jr., comes as Gilday is scheduled to appear in the courthouse Wednesday to be sentenced to 50 years in prison, a term reached by lawyers in exchange for Gilday’s guilty plea in the criminal case.

The wrongful death civil case was file on Aug. 12 on Donald Arsenault’s behalf by his attorney, James O’Connell III of the law firm Berman and Simmons, seeking an unspecified amount of money for the loss of his wife, including for her conscious pain and suffering before her death.

“The loss of Lynn Arsenault has been devastating to her husband, other family members, and her community,” O’Connell said in a written statement on Tuesday. “No one should have to endure what they have gone through over the past year. This lawsuit is intended to make sure that justice is delivered in Maine’s civil court system, just as it has been done in the criminal system.”

Gilday, 44, of Belfast, pleaded guilty on July 21 to a charge of murder for the killing of Lynn Arsenault, 55, and to charges of attempted murder and elevated aggravated assault for shooting her son, Mathew Day, now 23.

Under terms of the plea agreement, Gilday will be sentenced to 50 years for murdering Arsenault and 15 years for the attack on Day, to be served at the same time as the murder sentence. The agreement was reached after negotiations between the prosecutor, Assistant Attorney General Leane Zainea, and Gilday’s attorneys.

One of Gilday’s attorneys, Jeremy Pratt, said last month that Gilday does not plan to speak at the sentencing hearing, but planned instead to submit a letter to the court expressing his regrets.

No clear motive has ever been disclosed for why Gilday went to Day’s home and fired birdshot from a 12-gauge shotgun into Day’s arm and stomach, and into his mother’s shoulder and chest.

The civil case adds some answer to the mystery in suggesting that Gilday had no motive to shoot Lynn Arsenault, that her death was a mistake caused by his negligence and recklessness.

“Gilday attempted to enter Day’s house in a disoriented and angry state, with the intent of causing harm to Day, with whom he was angry due to personal issues,” O’Connell wrote in the three-page complaint. “Gilday discharged a firearm into Day’s home, inadvertently striking Lynn Arsenault, who was there visiting her son.”

Day, who was 22 at the time, lay bleeding on the kitchen floor but was able to identify Gilday as the gunman. He told police he didn’t know why Gilday had shot them, according to an affidavit filed in court last year by Maine State Police Detective Dean Jackson.

Half an hour before the shooting, Gilday had been at his home at 30A Springbrook Drive in Belfast, dressed in a bathrobe and crying at one point when an acquaintance, Samantha Ladd, stopped to visit him.

Gilday told her he was going to “shoot some people tonight,” but answered only “I don’t care” when Ladd asked whom he was going to shoot, according to an account she gave to police.

Gilday has been in custody since the day after the shootings, when police arrested him at Penobscot Bay Medical Center in Rockport. He had been admitted to the center’s unit for psychiatric and addiction patients on Aug. 29 after telling the staff that he was a drug addict who “had taken a large amount of opiates the night before,” Jackson said in the affidavit.

A possible motive for the shooting could hinge on Gilday’s relationship with Day’s girlfriend, Misty Linscott, and her mother, Linda Linscott. Gilday drove Misty Linscott to report for a jail term earlier in the day before the shooting.

Misty Linscott told Jackson that Gilday was just a friend who would give her rides, that they had done drugs together but that they were not in an intimate relationship, Jackson said in his affidavit.

“She described Gilday as being a little strange. He would stare at her and make inappropriate comments to her in front of Day,” Jackson wrote.

Linda Linscott told police that Misty had lost custody of her children to her, and that Gilday had become involved, helping Misty steal a camera from her with pictures that could be used to influence custody arguments, the affidavit says.

“According to Linda Linscott, Todd Gilday was upset because he seemed to think that she was trying to help the state take Misty’s kids away from her,” Jackson wrote.

The affidavit also provides detailed accounts from Day’s friend Jonathan Riley, who was in Arsenault’s house when Gilday fired the shots, and from a police interview with Day as he recovered at Eastern Maine Medical Center in Bangor.

Day told police that he considered Gilday a nice guy but “a bit odd.” He said he and Gilday acquired drugs for each other and Gilday gave him rides.

Gilday texted Day before the shooting, asking to talk to him about Misty and Linda Linscott’s custody dispute. Day agreed, but saw, when Gilday arrived at the house, that he had a shotgun with him.

“Mathew closed the door and tried to lock it, but Gilday shot through the door,” Jackson said in the affidavit.

Riley told police that he hid behind the couch in the living room but saw Gilday shoot Day, then Day’s mother as she came out of her bedroom, where she had been sleeping.

“Riley heard Mathew Day pleading with Todd Gilday not to kill them. Todd Gilday responded, ‘I’m going to kill everyone,’ ” Jackson wrote.

After the shootings, Gilday left the house and drove away, apparently without seeing Riley or firing another shot. Police found the shotgun on Sept. 1 in Levenseller Pond at the Searsmont-Lincolnville town line and traced it back to a Walmart in Bangor, where Gilday bought it in 2012, according to the affidavit.