SANTA CRUZ — A Santa Cruz surf instructor accused of seeking sex with underage girls and recording them in a dressing room could face federal charges, police said Monday.

The scope of the case against 38-year-old Dylan David Greiner widened during the weekend as police received calls from families as far away as New York, said Santa Cruz Deputy Police Chief Steve Clark.

“It”s a worst-case scenario,” Clark said. “We have someone who was popular in the community and had access to girls, and he exploited that.”

Greiner, owner of Santa Cruz Surf School, was arrested Friday on suspicion of two felony charges of lewd acts with children. He is expected to be arraigned Wednesday and remained in Santa Cruz County Jail on Monday in lieu of $500,000 bail, according to jail records.

A federal investigation could be launched if the case involves interstate victims and if the Santa Cruz County District Attorney”s Office relinquishes control, police said.

Two girls were 13 and 14 years old when they began relationships with Greiner, police said. When authorities questioned him, Greiner admitted to detectives that he made sexual advances to the girls and wrote apology letters to them and the community, Clark said.

An investigation determined Greiner also secretly recorded girls changing into wetsuits at his shop on Center Street, and he photographed and videotaped them with focuses on their private areas. His cellphone and at least two computers were seized from his home.

“This is a person who was in a position of trust, and he violated that trust,” Clark said.

Greiner — who had been in the public eye for a recent trash cleanup crusade at Cowell Beach and elsewhere — became the subject of criticism on Facebook this weekend.

Parent Gemma Locatelli said she had a bad feeling about Greiner when she and her then 14-year-old daughter asked about surf lessons about 18 months ago. Locatelli wanted to learn to surf with her daughter, but Greiner wouldn”t have it.

“He kept saying, ”Just drop her off, there”s lots of girls her age,”” Locatelli said. “He said, ”I”m really trustworthy, everybody trusts me.””

When she pushed to do the class together, Greiner got defensive, she said. He wouldn”t look her in the eye, and Locatelli did not enroll her daughter.

“I had a weird feeling. All the hairs on the back of my neck stood up,” Locatelli said.

She would advise other parents to trust their instincts and do their own informal background checks on anyone who will be with their children. Doing so could start on a Yelp page, for instance, where previous customers often scrutinize businesses.

Another red flag is when an instructor tries to separate parent from child, Locatelli said.

SURF SCHOOLS

Greiner”s surf school was one of four surf schools at Cowell Beach permitted by the city.

Recreation director Carol Scurich said Greiner”s permit remains active, meaning employees still can give instruction to clients. But she said the city attorney and police are looking into taking action on the permit.

There also are a handful of surf schools in Capitola and other areas of the county.

Rudy Escalante, the Capitola police chief, said the surf schools he knows in Capitola don”t have changing rooms. Most surfers simply use a towel to change into a wetsuit.

As a parent, Escalante echoed some of Locatelli”s advice about checking on adults who care for children and teens.

“I would encourage people to ask those pertinent questions to where they feel comfortable to do business with people,” Escalante said. “If I don”t feel comfortable, then it”s not happening.”

It”s also worth reminding parents to tell children it”s OK to say “no,” Escalante said, adding that the case is “a very sad situation.”

Sentinel staff writer J.M. Brown contributed to this report. Follow Sentinel reporter Stephen Baxter at Twitter.com/sbaxter_sc