The father of a missing Tucson girl claims at least one person knows something about her disappearance more than a year ago.Tuesday would be Isabel Celis' 8th birthday. Her father, Sergio Celis, said a family member he would not name has refused to talk to the family's private investigator."It's frustrating to run against a wall," Sergio Celis said. "This person has an attorney and they're not going to let them speak a word, protecting somebody that we feel has a lot of information to give. And if it has to be anonymous, so be it. We just want her home. We want our baby home. We want this nightmare over."Sergio Celis said he doesn't believe the person committed a crime, just that the person has information that could be critical to the case.Isabel disappeared from her family home 18 months ago.Police have said they believe she was abducted, but they have not named a suspect nor speculated where she could be.Her mother, Rebecca, posted to Facebook on Monday that the family will have a special mass Tuesday in honor of her daughter's 8th birthday. They have extended an invitation to the community to pray with the family at St. Joseph Roman Catholic Parish about 9 a.m.The family said it plans another mass on Sept. 1.Copyright 2013 CBS 5 (KPHO Broadcasting Corporation). All rights reserved.