LOWELL, Mass. — Disturbing new details revealed in court on Tuesday shed light on the death of Jill Kloppenberg, the missing Lowell woman whose remains were found buried beneath the concrete floor of a garage at a Tyngsborough home over the weekend.
The suspect in Kloppenberg‘s death, 40-year-old Sean Sullivan, was arraigned on a charge of murder in Lowell District Court, where a prosecutor outlined a chilling sequence of events that began more than a year ago.
A plea of not guilty was entered on behalf of Sullivan, and the judge ordered him held without bail.
It’s alleged that Sullivan told a friend in February 2026 that he had killed a woman he knew as Jill, claiming the shooting happened “accidentally” in February 2025.
Middlesex Assistant District Attorney Ceara Mahoney said Sullivan told his friend the gun went off while he was holding it and drifting to sleep next to Jill. The bullet struck her in the chest. Sullivan allegedly admitted that Kloppenberg “soon died,” and despite believing she might have survived, he chose not to call for help.
In the statement of facts read in court, Mahoney said Sullivan kept Jill’s body in his bedroom for several days before moving her to the garage of the Audrey Avenue home. There, investigators said, he:
- Dug a hole in the concrete floor
- Placed her body inside
- Sealed the hole with concrete and epoxy
The friend who heard Sullivan’s confession later contacted the Nashua Police Department, sparking the investigation.
Nashua police discovered that Tewksbury police had an active missing person case for Jill Kloppenberg, who had not been seen since January 2025 and was last spotted in Lowell.
Tewksbury detectives interviewed the friend, who repeated Sullivan’s alleged confession, “He accidentally shot somebody … a female named Jill.”
On March 15, 2026, Tyngsborough, Tewksbury, and Massachusetts State Police executed a search warrant at Sullivan’s home. Mahoney said investigators found:
- A 3-by‑5‑foot patched area of the garage floor
- An “anomaly” detected by ground‑penetrating radar
- A large wrapped object buried beneath the concrete
The Chief Medical Examiner confirmed the remains were human. Dental records later positively identified the body as Jill Kloppenberg.
Mahoney said that a preliminary autopsy revealed a through‑and‑through gunshot wound, with an entry wound to the chest and an exit wound to the back.
During the excavation, Sullivan spoke with investigators again. Mahoney said he admitted:
- He took a firearm out of his safe while Jill was visiting
- He “twitched,” causing the gun to fire
- He saw Jill collapse and determined she had died
- He buried her body under the garage floor
Sullivan’s attorney argued in court that the evidence points to a case of involuntary manslaughter, not murder, calling the shooting a “tragic accident followed by inexcusable panic.”
He is due back in court for a probable cause hearing on April 17.
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