FARMINGTON — A Shiprock man pleaded guilty in federal court Wednesday to sexually assaulting his girlfriend's granddaughter.

Leo Thompson, 54, will be sentenced to eight years in prison and be required to register as a sex offender as a condition of his plea agreement, according to a news release from the U.S. Attorney's Office for the District of New Mexico.

The FBI and Navajo Nation criminal investigators were called to Northern Navajo Medical Center on Nov. 27, 2012, when a 13-year-old girl reported that Thompson had sexually assaulted her. The girl told the investigators Thompson had raped her on regular basis since she was 7 years old, according to court documents.

Thompson pleaded guilty to a sex crime against the girl that happened in November 2012.

The assault Thompson pleaded to -- and the other allegations against him -- happened when the girl was living at her grandmother's house at the East Low Rent apartments in Shiprock, according to court documents.

Thompson was arrested Jan. 29, 2013.

In addition to the victim's statements, investigators also spoke to the victim's younger sisters, who reported seeing the girl be abused.

The U.S. Attorney's Office said the case was prosecuted as part of Project Safe Childhood, a nationwide initiative the U.S. Department of Justice launched in May 2006 to combat child sexual exploitation and abuse.

Ryan Boetel covers crime for The Daily Times. He can be reached at 505-564-4644 and rboetel@daily-times.com. Follow him on Twitter @rboetel on Twitter.