A Los Angeles County grand jury has indicted a former El Segundo teacher on suspicion of molesting 13 boys in three counties.

Jeffrey Thomas Simonek, 28, faces 20 counts of lewd acts upon a child under 14 years old, according to authorities. He was arrested Nov. 26 and remains in county jail in lieu of $20-million bail.

According to the indictment unsealed Dec. 12, the alleged crimes occurred between April 2006 and June 2012, with most victims molested multiple times over spans of a few days to several months.

Investigators allege 15 of the felony counts were committed in Los Angeles County, three in Plumas County and two in Orange County, according to the indictment.

Simonek became the target of an investigation Aug. 2, 2012, when El Segundo police received an anonymous report through the county Department of Children and Family Services that two fifth-grade boys where Simonek taught — Center Street School — had been assaulted, according to a statement from the department.

Simonek was placed on administrative leave during the investigation. He was arrested on Oct. 24, 2012, and released on bail.

The case was presented to prosecutors, but they did not file charges. A spokesperson could not be reached for comment.

The case expanded when investigators learned that Simonek had access to hundreds of other children through work as a counselor at a Lake Tahoe-area summer camp that drew boys ages 6 to 11 from 2005 to 2011, Lt. Raymond Garcia told The Times.

Investigators allege that he committed similar crimes at the camp, according to a police statement.

Simonek also volunteered at the Painted Turtle, a Lake Hughes youth camp for children with serious medical conditions. It was co-founded by actor Paul Newman, the South Bay Conservatory in Torrance, the now-defunct RnB Dance Studio in Torrance and the Big Brothers Big Sisters of Orange County, Garcia said.

Simonek was arraigned Dec. 12, the same day the indictment was unsealed. He has pleaded not guilty to the charges. His next court appearance is scheduled in February.

ruben.vives@latimes.com