Michael Kellar (L), 56, and Gail Burnworth (R), 50, are pictured in this undated handout photo obtained by Reuters August 8, 2017. San Jose Police Department/Handout via REUTERS

Impromptu detective work by a woman on an airline flight resulted in the arrest of another passenger whose text messages about sexually abusing children she photographed and gave to police, federal prosecutors said on Tuesday.

Michael Kellar of Tacoma, Washington, will be charged on Tuesday with conspiring to make child pornography, prosecutors said, after he exchanged the explicit messages with a woman he met a year ago on a dating website.

He sent the messages to Gail Lynn Burnworth, also of Tacoma, while he was flying from Seattle to San Jose, California, on July 31, according to a criminal complaint.

The woman behind Kellar became alarmed as she could see the couple describe sexually abusing two children in Burnworth's care. Burnworth promised to send Kellar pictures of her molesting the children while they slept.

"You can do this or are you just saying this???" Kellar replied in one message, apparently unaware the passenger behind him was taking photographs of his phone's screen.

"No I think I can do it if I don't have parents over my shoulder," Burnworth responded. "And then on Sunday, I will have the kids no parents just kids."

The alarmed passenger, who has not been named, told flight attendants and later shared her photos with police who arrived to meet Kellar at the airport, the complaint said.

Kellar gave police his phone and told them he "likely" had a sexual interest in children, according to the complaint, but that the discussion of abusing the children in Burnworth's care was fantasy. Kellar remained in custody on Tuesday and it was not clear whether he had a lawyer.

Later that day, police arrested Burnworth, who also acknowledged a sexual interest in prepubescent boys and girls, the complaint said.

Burnworth lives in Tacoma, Washington, with a man she referred to in messages as her former husband, along with his three children and the children's mother.

She told authorities she had made up to 20 sexual images of the youngest two, a boy and a girl under 12 years old, to send to Kellar.

On her phone, police saw the full-text message exchange in which the couple discussed drugging the children in order to rape them while the parents were away.

Both Kellar and Burnworth, who appeared in federal court in Tacoma on Monday, are charged with conspiring to produce child pornography, while Kellar, who is due to appear in federal court in San Jose, also faces a charge of attempted enticement of a minor.

Burnworth's lawyer, a public defender, did not respond to a request for comment.