LINCOLN, Neb. (AP) — The children of a woman killed in a house explosion last year are accusing the city of Lincoln, Lancaster County and gas utility Black Hills Energy of negligently causing her death.

The Lincoln Journal Star reports that Amy Roche and Matt Jasa have filed a more than $4 million tort claim in the blast that killed their mother, Jeanne Jasa. The filing is required before a lawsuit can be filed against a Nebraska government agency.

Lincoln police are still investigating the August 2017 natural gas explosion and haven’t yet ruled out foul play.

The family alleges Black Hills infrastructure and its safety devices failed, allowing gas to fill the home.

A utility spokeswoman declined to comment. Assistant City Attorney Elizabeth Elliott says she hasn’t made a formal decision on the claim.