Brock Turner, a one-time college swimming star whose six-month sentence for a sexual assault conviction sparked national outrage, appealed his guilty verdict Friday.

In their filing, Turner's attorneys said their client was denied due process at a trial they described as a "detailed and lengthy set of lies."

"What we are saying that what happened is not a crime," John Tompkins, Turner's legal adviser, told KNTV. "It happened, but it was not anywhere close to a crime."

In addition to overturning the guilty verdict, Turner's lawyers also hope to overturn his mandatory lifetime requirement to register as a sex offender.

Turner was convicted of three felony counts of sexual assault after he was accused of raping a woman outside a Stanford fraternity party in January 2015. Santa Clara County Judge Aaron Persky sentenced him to six months in jail, citing the "extraordinary circumstances" of Turner's youth, clean criminal record and other considerations in departing from the minimum sentence of two years in prison.

Prosecutors had asked for a six-year prison sentence. Facing a recall push, Persky voluntarily removed himself from hearing criminal cases.

The case drew national attention after the victim's 7,200-word letter to Turner that she read in the courtroom during sentencing was published online.

Turner was released from prison in September 2016 and returned to his home state of Ohio, where he registered as a sex offender. He is required to register as a Tier III sex offender each 90 days for the rest of his life.

The Associated Press contributed to this report.