Police were on the lookout Tuesday for two young men who grabbed two teenage girls in a dark Rancho Penasquitos park over the Labor Day weekend, threatened them with a knife and sexually assaulted them.The victims, a 15-year-old and a 16-year-old who both live in Clairemont, were attending a family party in the 12600 block of Spindletop Road on Saturday evening when they decided to take a walk into a nearby open-space area, according to San Diego police.They were sitting beneath a tree about 8:15 p.m. when two men who appeared to be in their late teens or early 20s walked by them, San Diego Police Department public affairs Lt. Andra Brown said.Moments later, the strangers grabbed the girls from behind, and one put a knife to the older victim's throat while his cohort put his hand over the other one's mouth.The assailants then forced the girls into a more secluded part of the park and sexually assaulted them for 20 to 30 minutes, Brown said. The attackers finally fled when a parent of one victim came looking for the girls.Hearing the adult calling out the girls' names, the men ordered them to walk away, then climbed onto a single bicycle and rode away.The girls described the assailants, who spoke English to them and Spanish between themselves, as clean-shaven, roughly 5-foot-6-inch Hispanic males between 18 to 20 years old, Brown said. One was wearing a baseball cap and a white shirt with a San Diego Padres logo on it, and the other had on a shirt with an attached hood.A resident told 10News he heard a police helicopter above the neighborhood that night."The helicopter announced over the speaker that two girls had been raped," he said.A white picket fence separates Weixing Chen's house from the dense brush where police said the girls were sexually assaulted."It was very scary that it happened maybe 30 [or] 40 yards from our house," said Chen.Chen's daughter Connie, who is also 15, went straight to the Internet."I was on Facebook and everyone had updates about it," she said.Though she was feeling scared and vulnerable, she found comfort talking to friends about it."Having someone that close to my age being able to relate to that It's really scary because you never really think this would happen in your neighborhood and it does," she said.Brown told 10News the information going public was delayed due to conflicting information from the victims about the assailants. She also said there was not enough information gathered at the time to release it.Brown acknowledged the immediate area was informed by the helicopter announcements.Police have not received any reports of similar recent assaults in the area, according to Brown."It looks like it was a crime of opportunity," Brown said. "We're not looking at this as series-related."Investigators were working with the victims to generate composite facial renderings of the assailants to release the public.Members of the San Diego Police Department will hold a meeting at the Doubletree resort in Rancho Penasquitos on Wednesday at 7:30 p.m. to discuss the incident with residents and to keep them informed of the efforts to find the attackers.