DENVER -- A Grand Junction man faces 13 felony counts after investigators said four children of a family friend claimed he sexually assaulted them.

David Diesburg, 22, was arrested Monday by Mesa County investigators on a warrant charging him with four counts of sex assault on a child with a pattern of abuse, four counts of sex assault on a child by a person in a position of trust, four counts of sex assault on a child -- no force, and one count of sex assault on a child -- with force.

Investigators said Diesburg was a "family friend" of a divorced woman who had recently moved from Iowa to Grand Junction and he baby-sat her four girls, all under the age of 11, from time to time.

Deputies were dispatched to St. Mary's Medical Center on Sunday to investigate a complaint of sexual abuse of four children. The girls' mother, who TheDenverChannel is not identifying to protect the identities of the children, told deputies she had left her children with Diesburg while she returned briefly to Iowa and became suspicious that they may have been sexually abused.

The children, whose ages weren't released, related they had played a game called "humping tag" with Diesburg on a number of occasions.

When questioned, the children related they were sexually assaulted by Diesburg in addition to playing the game, and graphically described the alleged assaults.

One of the girls said that the most recent assault occurred on Saturday, when Diesburg held her down and had sexual intercourse with her.

According to the arrest affidavit, Diesburg was questioned by investigators and admitted he sexually assaulted the girl. He also admitted playing "humping tag" with the children at least 50 times, the affidavit stated.

Diesburg is being held without bail, the Mesa County Sheriff's Office said in a news release on Tuesday.