The family of a women's rights activist from Uganda sued the National Park Service this month after she was decapitated last year by a gate at Utah's Arches National Park.

The gate had been left unlatched against federal policy for two weeks before it struck Esther Nakajjigo in June 2020, according to the lawsuit filed in Denver.

She and her husband were newlyweds traveling in the well-known park when the wind caught the gate as they drove out, Fox13-KSTU in Salt Lake City reported.

The lawsuit does not specify the amount of damages being sought, but Nakajjigo's family has previously filed a US$270 million ($379 million) notice of claim. Notices of claim must be filed ahead of lawsuits against government agencies and the lawsuit was filed June 8 in federal court.

The gate sliced through the side of their rented car, striking Nakajjigo in the head and neck and killing her, the lawsuit said.

Attorney, Deborah Chang told local news that the couple had no chance of detecting the lethal object.

"It is like a metal spear or a lance," said Chang, "and it is at its most narrow profile pointing towards him in adobe colors like the surrounding area.

The couple's Malibu car was hit by a metal gate, which was allegedly unlatched. ( Photo / Supplied)

"You wouldn't able to detect it or see it."

Esther's husband Ludo Michaud witnessed his wife's death, something he has called the "worst thing I hope I will ever see."

Michaud, a French national, reported leaping from the car in search of help. A bystander from the visitor centre observed that Michaud was in such a hurry the car continued to move, having failed to put brakes on the car.

The family of a women's rights activist from decapitated at Utah's Arches National Park is suing the US service. (Photo / AP)

He said the episode was caught on video and would show they were helpless to act.

"I think the video will show in detail how fast this happened," Chang said.

"Literally a split second," she added, "giving Ludo absolutely zero chance to avoid it."

"I really wanted to show her Arches," he told KSTU, "because I know all the parks around Moab … It's one of my favorite places in the US, if not my favorite place."

'The worst thing': Memorial services held for Esther Nakajjigo, Uganda's ambassador for Women & Girls. (Photo / Supplied)

Nakajjigo, 25, was born in Kampala, Uganda, and used her university tuition money to start a nonprofit community health care center for girls and young women when she was a teenager.

She earned numerous humanitarian awards and created a popular reality television series aimed at empowering young mothers. She was attending a social-entrepreneurship program in Colorado at the time of her death.

A National Park Service spokesperson declined comment Monday on the lawsuit, Fox13-KSTU reported. The park service previously issued a statement expressing sympathy to her family.

- Associated Press