Michael Kellar, 58, was convicted of conspiracy to produce child pornography and was sentenced to 15 years in prison on Friday

A Washington man who was seen texting someone about molesting children while on a plane to California, has been sentenced to 15 years in prison.

Michael Kellar, 58, was convicted of conspiracy to produce child pornography, according to a release from the US Attorney's Office in Washington's Western District.

Kellar's girlfriend, 52-year-old Lynn Burnworth, pleaded guilty to distribution of child pornography. She will be sentenced on March 27.

The pair were arrested in August 2017 after a passenger noticed the exchange between the two on July 31 of that year.

Burnworth was in Tacoma, Washington, at the time of the flight. An investigation found that the two had produced and shared sexually explicit images of children that were in the woman's care.

'This case for me ... has been the most disturbing case that I have had…. (These offenses) beset on society a waterfall of negative repercussions that can never be repaired,' US District Judge Ronald B. Leighton said during Kellar's sentencing.

US Attorney Brian T Moran praised the airline passenger who came forward about the 'disturbing texts she witnessed on her flight.'

Kellar was arrested at Mineta San Jose International Airport after a schoolteacher behind him spotted the texts and notified staff. California police then liaised with colleagues in Washington to find his girlfriend

'We all have had that moment when we question: "Do I get involved,"' Moran continued. 'In this case, that witness triggered the rescue of two small children and the investigation that revealed this defendant to be a predator – collecting images of child sexual abuse.'

After witnessing the messages on Kellar's device, the passenger informed airline flight attendants who then had law enforcement meet the plane.

Kellar's girlfriend, 52-year-old Lynn Burnworth, pleaded guilty to distribution of child pornography. She will be sentenced on March 27

Kellar was arrested by police at the scene and authorities discovered the graphic explicit images he exchanged with Burnworth.

A court-authorized warrant was obtained to search Burnworth's residence, where investigators found sexually explicit images on her phone and several hundred images of child pornography on Kellar's laptop.

'Sexual exploitation of children is particularly heinous, and the FBI has made it a priority to hold those who choose to perpetrate these acts accountable,' said Special Agent in Charge Raymond Duda, FBI-Seattle.

'Mr. Kellar will now answer for his crimes against some of the most vulnerable in our community. This case demonstrates the FBI will rigorously investigate these crimes and reinforces that victimizing children will not be tolerated by law enforcement.'

Kellar pleaded guilty to the charge and access with intent to view child pornography on September 25, 2019. Once he is released from prison, Kellar will have to be on lifetime supervised release and will need to register as a sex offender.