An explosive device damaged a Greenfield family’s Jeep in 2018, as seen in this image from an FBI affidavit, and led to their neighbor’s arrest.

A man who detonated an explosive device in a neighbor’s parked vehicle in Greenfield in late 2018 was sentenced to probation Tuesday, after his attorney and the prosecutor agreed that mental-health treatment, rather than prison, would be most appropriate.

Alexander Arsenault was sentenced on one count of possession of an unregistered firearm, defined in federal law to include explosives, which he pleaded guilty to in U.S. District Court in Concord in January.

Government officials said Arsenault created “a homemade bomb consisting of a low explosive main charge and a non-electrical fusing system” and planted it in his neighbor’s unoccupied Jeep.

The explosion on the morning of Dec. 1, 2018, woke the family that owned the Jeep, according to an FBI agent’s affidavit. The family, the Conways, reported that they had not received any threats, but Arsenault had previously had “issues” with their children four-wheeling.

A tipster later informed law enforcement that Arsenault was acquiring materials to create small explosives for target practice, and Greenfield police were receiving complaints of loud noises, thought to be explosions, in the area, Agent Shayne Tongbua wrote in the affidavit.

While advisory sentencing guidelines would typically call for a sentence of about three to four years of incarceration, the prosecution and defense agreed that probation, rather than prison time, was warranted due to the condition of Arsenault’s mental health.

At the time of the December 2018 incident, Arsenault’s “mental health had substantially deteriorated, and he was suffering from psychotic episodes in which he irrationally believed that he was being harassed,” Assistant U.S. Attorney Anna Krasinski wrote in a sentencing memorandum.

The victims agreed to the sentence, Krasinski wrote. While Arsenault’s actions were dangerous and caused them fear, the victims recognized that incarceration “would, in the long run, likely increase the defendant’s danger to the community rather than lessen it,” she wrote. “The victims also recognize that the key to rehabilitation and community protection here lies in consistent mental health treatment for the defendant.”

Arsenault had been free on bail while his case was pending.

He has been engaging in mental-health treatment since last fall and has “markedly improved,” his attorney, Behzad Mirhashem, wrote in a sentencing memo. He added that Arsenault has a treatment plan in place and a strong support network, which would not be available in prison.

“With proper treatment, there is no reason to expect that his conduct will be repeated,” Mirhashem wrote.

Reached Thursday, Mirhashem declined to comment further.

In addition to continuing mental-health treatment, Arsenault was also required to pay about $4,500 in restitution for the damage to the Jeep, have no contact with his former neighbors and not live at the Greenfield house.

Paul Cuno-Booth can be reached at 352-1234, extension 1409, or pbooth@keenesentinel.com. Follow him on Twitter @PCunoBoothKS.