"The FBI is looking at it and they're saying it is a criminal matter," Sacco family spokeswoman Aileen Barry told AOL News. "They've asked us to hold off so they can get their people in line, so we can't say a lot about what they're doing. But honestly we don't know a lot."
Sacco was last heard from on April 20, when she set out on a weeklong hike in the Langtang Valley just south of the Tibetan border. The Colorado native was familiar with the area and had gone hiking there in the past.
When Sacco failed to make contact with her family by May 4, they contacted the U.S. Embassy in Nepal for assistance. Officials were able to confirm she had made it to two check-in stations along the Langtang Valley trail, but whatever happened after that remains a mystery.
Last month, Sacco's father, Paul Sacco, flew to Nepal along with her brother, Crofton, and Barry's nephew, Dinesh Shakya, an expert on Langtang National Park. The trio remained there until last week.
"They walked the trails, went up in helicopters, out with the search teams, but nothing," Barry said. "They found nothing."
The FBI is revealing few details to the family, but Barry says they do know agents are reviewing lists of hikers who signed in on the trail around the time Sacco went missing. Calls by AOL News to the FBI office in Denver were transferred to the State Department, but have yet to be returned.
"They have trekking numbers and passport numbers for those people, so they are meticulously going through each of those people," Barry said. "I know some are from Hungary, Israel and the United States."
In addition to the FBI, the Sacco family has its own team of private investigators looking into the case. They have turned up a few leads, Barry says, but are having difficulty getting locals to confide in them.
"There have been Nepalese people who have said, 'We know where she is and you'll never find her.' "
Barry explained some residents may be tight-lipped because of an incident that officials at the U.S. Embassy in Nepal related to the Sacco family.
"The embassy told us that 15 years ago there were two fishermen who found a body in a river," Barry explained. "These fishermen went to the authorities and told them. Afterward, they were arrested. There was no trial and they were thrown in prison. They just got out, so that is on everyone's mind and they aren't talking."
In an effort to help loosen tongues, the Sacco family has increased the reward for information from 100,000 Nepali rupees (about $1,300) to 300,000 rupees (about $4,000). It's a significant sum in Nepal, where the average income is less than $200 a year.
The family wanted to offer more money, but officials at the embassy asked them not to, for fear of "endangering other tourists," Barry said.
At this point in the case, kidnapping for ransom seems unlikely, as no one has attempted to contact the Sacco family. As a result, they are concerned she may have been kidnapped by members of an underground sex-trade ring. According to the U.S. State Department, between 10,000 and 15,000 women and girls from Nepal are sold into sexual slavery every year.
"The tourism department and embassy says it wouldn't happen to a Westerner," Barry said, "but how do we know that?"
As a result of ongoing political violence and the "harassment of female travelers" in Nepal, the State Department issued a warning to U.S. citizens earlier this month, alerting them to the potential risks of traveling there.
"The U.S. Embassy in Kathmandu strongly recommends that you do not hike alone or become separated from larger traveling parties while on a trail," the warning read. "Solo trekking is dangerous, has contributed to injuries and deaths, and makes an individual more vulnerable to criminals. Foreign trekkers have gone missing while trekking alone."
Meanwhile, Barry said, the Saccos will continue doing everything in their power to locate their daughter and will not stop until she is found -- dead or alive.
"All of us have committed that we are going to find her. We know it's a crime and something happened to her," she said. "She didn't just disappear."