SAN JOSE -- A registered sex offender convicted of molesting a young relative was caught volunteering at a San Jose Catholic elementary school festival in October but was not arrested after the sheriff's department learned he had written permission from a church official to be there, this newspaper has learned.

Mark Christopher Gurries, 51, was volunteering at the St. Frances Cabrini Catholic Elementary School and parish festival on Oct. 6 when someone recognized him as a sex offender and informed a deputy sheriff who was working as security, the Santa Clara County Sheriff's Office confirmed.

How Gurries got the letter and its authenticity are uncertain. The pastor who oversees the school said he knew nothing about Gurries' conviction or the letter until the night of the festival. A spokeswoman for the Diocese of San Jose said as of late Wednesday afternoon, the diocese had not been able to track the source of the letter.

The Sheriff's Office is continuing to investigate the incident, including trying to nail down whether the letter is valid. There were no complaints about Gurries' behavior that night.

One parent who attended the festival told this newspaper that Gurries was working the sound system on the night of the parish festival and had little or no supervision. The festival was geared for the school's kindergarten through eighth-grade students and church parishioners. The parent, who spoke anonymously for fear of possible retribution from school and church officials, said there were several hundred kids in attendance.

"We want to know why they put our kids in danger that night," the parent said. "It's a slap in the face to parents who think their children are protected. That makes me want to leave the church and the school and go elsewhere."

Gurries, who was convicted in February 2010 of one count of lewd and lascivious acts on a child under the age of 14, was not arrested because of the existence of a letter signed by a church official allowing him to be on school grounds, according to sheriff's spokesman Sgt. Jose Cardoza. But the deputy escorted him off campus "out of an abundance of caution."

The letter indicated that the church was aware of Gurries' status as a registered sex offender, according to the Sheriff's Office. The letter also indicated Gurries is allowed to be on school campus and participate in parent functions at the school and parish, and needs to be supervised when children are present.

The California Penal Code states that a registered sex offender may be allowed on school grounds if they have written permission from the chief administrative official of that school, Cardoza said. Without such permission, a registered sex offender can be convicted of a misdemeanor for being on school grounds.

The existence of the letter meant the Sheriff's Office could not make a criminal arrest, Cardoza said.

The deputy was hired by the school as an off-duty assignment to provide extra security for the large event, Cardoza said. The school and church are located on Woodward Road in an unincorporated part of San Jose and fall under the jurisdiction of the Sheriff's Office.

A report on the incident was forwarded to the Santa Clara County Probation Department, which did not return phone calls seeking comment about the letter. The Sheriff's Office did not release the letter or provide information about who signed it.

The Rev. Lieu Vu, pastor at St. Frances Cabrini, said Wednesday he did not hear about the letter until the night of the festival.

"I have no idea about that letter," Vu said. "I did not write any letter."

Vu said a parishioner told him about Gurries' criminal background on the night of the festival and showed him a mug shot with information on the state Department of Justice Megan's Law website. Vu said he decided to let the deputy handle the situation.

A spokeswoman for the Diocese of San Jose, the Catholic Church's governing body in Santa Clara County, said she made inquiries about the letter but as of 5 p.m. Wednesday did not have any information about the source of the letter.

Reached by telephone, Gurries said he would "let the church decide whether they release the information" and said the letter stands on its own.

Gurries declined to comment about the letter and his sex abuse conviction.

Vu said the school did not notify parents that a registered sex offender had been on campus.

Court records indicate that Gurries was sentenced on May 17, 2010, after reaching a plea deal with the Santa Clara County District Attorney's Office regarding charges of molesting a young relative. Gurries was originally charged with six counts of lewd and lascivious acts on a child under the age of 14, stemming from allegations of repeated molestation of a young relative.

The molestation happened more than 10 years ago, but the victim did not come forward to authorities until December 2008.

San Jose police recorded a phone conversation between Gurries and the victim in which he detailed the sex crimes, according to a police report. In the recorded conversation, Gurries told the victim specifically what he had done to her as a young child, including acts of oral copulation.

During the conversation, Gurries was asked why he did it. He replied: "Because I was a weak person. I don't know how else to say it. I felt lonely. I took advantage of you."

Gurries told detectives the abuse occurred when the child was 4 or 5 and he remembered 10 separate incidents with the victim.

The District Attorney's Office reached a deal with Gurries when the victim refused to testify, according to prosecutor Bill Butler. Gurries was sentenced to one year in county jail and was placed on three years' formal probation. As a condition of his probation, he is not to "work, be self-employed nor do volunteer work in an organization that involves supervision of children under 18 years, according to a court sentencing document.

"We were frustrated," Butler said. "We wanted to prosecute him fully. Still to this day it bugs me that we were not able to do more at that time ... because we realized the punishment was not commensurate with the crime."

Contact Mark Gomez at 408-920-5869. Follow him at Twitter.com/markmgomez.