An Indiana megachurch is to be sued over the death of an 18-year-old woman found unconscious in the church pastor's swimming pool.

Domonique "Nikki" Smith died on June 1 2015. She never regained consciousness after being found floating face down in the swimming pool at her pastor's home. Smith's family are now suing that pastor's church, ABC7 Eyewitness News reports.

"We need the closure. I need to know my baby's life was not taken in vain," said Vicki Walker, Smith's mother.

On Thursday Smith's parents filed a wrongful death lawsuit against the megachurch Family Christian Centre, and its pastors Steve and Melodye Munsey. Smith had been babysitting the Munsey's 6-year-old granddaughter.

Max Solomon, Smith's family's attorney, said: "The more we looked into it, the more we asked questions, the more questions we had."

The suit filed in Lake County Indiana civil court accuses the ministers of the Munster, Northwest Indiana megachurch of negligence. Smith was found unconscious on 29 May 2015, and died on 1 June 2015. Melodye Munsey and another woman had been at the Munseys' home at the time of the incident. "She should have been watching my child, while [Smith] watched their child," Walker said, according to the Northwest Indiana Times.

Walker's local counsel Trent A McCain said: "Instead of attending to her immediately, she [Munsey] made a judgement call and said, her soul has left her body, there's no sense of urgency."

The Lake County coroner had ruled Smith's death an accidental drowning, although no water was present in her lungs. Toxicology tests showed no signs of drugs or alcohol. Because Smith's organs were donated, there was no autopsy.

Smith's family decided to take legal action after seeing security video footage from the day Smith was found, which Schererville police acquired from the Munsey home. The family say it doesn't show everything and question an alleged 15-minute gap in the footage.

"There were a lot of things that looked very suspicious," said Walker. "I believe the tape was altered."

The Church has declined to comment on the lawsuit, while the Munsey family's attorney said on Thursday that he had not received a copy of the civil complaint and could not yet comment.