A Nashua man is being held on preventative detention after allegedly exposing himself to two girls and propositioning two others in a city park.Keith Bernasconi is charged with two counts of attempted felonious sexual assault and breach of bail conditions.Investigators said the first incident occurred Wednesday on a trail in Mine Falls Park, according to a police affidavit. Two members of a girls cross-country team reported that a man exposed himself to them, triggering the investigation."The girls that were involved with this were very brave," said Lt. Peter Urban. "They did the exact right thing going forward to their coaches and adults, reporting the incident that occurred in the park."In the same area of the park on Thursday, two teenage girls reported that a man asked them if they wanted to have sex, according to the affidavit."We conducted a series of interviews regarding the four juveniles that were involved," Urban said. "Based on their descriptions, we were able to identify Bernasconi as the offender."Bernasconi denied being involved, but a search of his residence also turned up some of the clothing that the girls described, police said.Police said they are concerned there might be other victims out there."If anyone had any other incidents in and around this time, we would ask them to come forward," Urban said.If Bernasconi makes bail, part of his conditions include having no unsupervised contact with minors. He is scheduled to be back in court on Nov. 29.