An expert witness told jurors Tuesday that he believes Linsday Clancy likely "retained an awareness of the illegality of killing" her children and is "criminally responsible" for her actions.

Forensic psychologist Kirk Heilbrun told jurors he believed Clancy did not experience an episode of psychosis when she strangled her children on the evening of Jan. 24, 2023, but did think she believed the act would prevent her children from future suffering by killing them before attempting to take her own life.

Heilbrun, who interviewed Clancy several times after the killings and has reviewed her medical records and clinicians' notes, testified that in his opinion Clancy justified "killing the children out of love" because she wanted them to avoid the "suffering" of growing up without a mother.

"[S]he retained an awareness of the illegality of killing others, including killing her children," Heilbrun testified. "I'm not the decision maker here, but my clinical opinion is that she was criminally responsible on Jan. 24, [2023]."

Heilbrun testified that, in his opinion, Clancy's actions could be best understood "as a serious suicide attempt combined with altruistic filicide, killing her children out of love, rather than as a response to command hallucinations."

While Heilbrun acknowledged that there were, in his opinion, some signs Clancy experienced psychosis, he argued that evidence suggests Clancy was aware of her actions that night and was experiencing "peritraumatic dissociation."

Heilbrun also tried to cast doubt on the defense's psychosis argument by highlighting slight "inconsistencies" in Clancy's account of the events and the number of individual decisions she needed to make to kill the children over the course of what the prosecution argues was 20 minutes.

Heilbrun added that based on his interviews with Clancy, he believes the 36-year-old is "still grieving" her children.

"She's still in mourning for those kids, and she used language like, 'I have lost everything, and I don't want to be here,'" he said. "So, I would not think of Lindsay Clancy as someone who is really self-centered or oriented only to herself. She is grieving her children."

Prosecutors in the murder trial of Lindsay Clancy plan to rest their rebuttal case later Tuesday after calling two final witnesses to cast doubt on the claim that the Massachusetts mother was in the throes of psychosis when she killed her three children.

Jurors on Monday were sent home early after testimony from a prosecution expert about Clancy’s religion prompted defense attorney Kevin Reddington to unsuccessfully move for a mistrial.

The request for a mistrial stemmed from the testimony of forensic psychologist Kirk Heilbrun, who tried to cast doubt on the claim that Clancy was hallucinating when she killed her children. Instead, he testified that the Clancy committed “altruistic filicide” in which, he testified that, she believed she was helping her children avoid suffering by killing them before taking her own life.

Heilbrun testified that Clancy may have believed that "she and the children would be together in heaven with God" if she killed them before taking her own life.

"She was raised as a Catholic, and this, as I understand, Catholic conditions, that's not necessarily what happens. But when I ask her about that, when I said, ‘Isn't suicide a mortal sin?'" Heilbrun said before he was interrupted by the judge.

Defense attorney Kevin Reddington argued that the testimony was "intentional misconduct on the part of the prosecution" and mentioned another incident last week when Clancy’s former mother-in-law was asked if "you’re aware that murder is considered a mortal sin?"

Prosecutors denied that the testimony about Clancy’s faith was intentional, and Judge Sullivan denied the motion for a mistrial, instead opting to instruct jurors to ignore the testimony.

Judge William Sullivan has denied a request by Lindsay Clancy's attorney Kevin Reddington to declare a mistrial, citing prosecutors' mention of Clancy's religion during recent testimony.

"I am not going to declare a mistrial at this time," Sullivan said.

Reddington had accused prosecutors of "intentional misconduct" after they asked forensic psychologist Kirk Heilbrun about a conversation he testified he had with Clancy, during which Heilbrun said they discussed her Catholic upbringing and the notion of suicide as a "mortal sin."

The phrase "mortal sin" has come up previously in this trial -- Clancy's former mother-in-law, an active churchgoer, was asked by prosecutors last week if she was "aware that murder is considered a mortal sin" -- with Sullivan instructing jurors at the time "not to consider" it "for any reason or any purpose."

At the time, Reddington did not move for a mistrial, calling it a "tactical decision," but said Monday that the most recent example was too much.

"It's the same thing again: talking about religion, talking about heaven, talking about killing people and the impact or the effect that it would have on Lindsay, as well as her children," he said.

Prosecutors pushed back on Reddington's motion for a mistrial Monday, arguing they did not intentionally invoke Clancy's religion, and that the remarks were relevant to the witness's testimony.

“There was no intentional conduct by the Commonwealth," prosecutor Jennifer Sprague told the judge. "That was not where I intended to go with this questioning, and I don't think that the witness intended to do any harm either, he was just explaining his rationale."

After denying the defense's request for a mistrial, Sullivan again instructed jurors to ignore recent testimony related to Clancy's religion, calling it "an inappropriate area of inquiry" and saying Heilbrun's "understanding of Catholic dogma or Catholic teaching is irrelevant."

Sullivan then sent the jury home for the day.

Testimony will resume with the same witness tomorrow morning at 9 a.m. ET.

For their second rebuttal witness, prosecutors called forensic psychologist Kirk Heilbrun to testify about his evaluation of Lindsay Clancy.

Heilbrun, who told jurors he met with Clancy four times this year and reviewed her medical records and clinicians' notes, testified that Clancy suffered from the side effects of the medications she was prescribed and that her medical providers disagreed on her diagnosis.

Heilbrun also testified that he believed Clancy suffered from bipolar II disorder, but testified that he did not observe manic episodes.

This was one of several competing assessments detailed by prosecutors and defense lawyers, with prosecutors pushing back on the bipolar diagnosis and defense lawyers arguing Clancy suffered from bipolar disorder with postpartum psychosis.

Heilbrun testified that Clancy began taking Zoloft, an antidepressant used to treat major depressive disorder, in October 2022, just a few months before she killed her children, at which point "things started getting very difficult."

Heilbrun told jurors that Clancy's anxiety got measurably worse after taking the medication and she continued to suffer from insomnia.

"Late in November [2022], she was still having a lot of trouble sleeping. She thought that she was close to what she called the end of her rope. She was feeling off and disconnected and sort of like a zombie," he testified.

He testified that Clancy began to suffer "horrible intrusive thoughts" in December 2022 -- the month before the killings -- including thoughts of potentially harming her children.

However, Heilbrun testified that Clancy's intrusive thoughts were not "auditory hallucinations" as alleged by defense lawyers.

Heilbrun also told jurors that Clancy had "a small shred of hope" following a brief inpatient stay in January 2023, but that her conditions eventually worsened.

"Over the course of the time that she was going through these experiences, between mid-October [2022] and late January [2023], she'd had a lot of providers offer a lot of diagnoses, and there were a lot of complicating considerations," he testified.