Dave Mustaine Megadeth front man is looking for his missing mother-in law. His mother-in-law has been suffering from Alzheimer's disease and has been missing for nearly three weeks.

The distraught singer is hopeful that someone may have just picked her up. That way, once they turn her in, there will be no questions asked. "We are not interested in prosecuting anyone," he told PEOPLE. "If anyone has taken her and is afraid to turn her in, no one is going to punish them. We just want to find out where she's at."

It has now been 20 days since the 76-year-old Sally Estabrook first went missing from the Pinezanita RV park and campground, which is located near Julian, California, an area that is 60 miles to the east of San Diego.

Estabrook was on the grounds with her husband, who went to take a shower. When he returned to where he left his wife, she was gone. Dave Mustaine said, "He's pretty upset, and rightly so. Her Alzheimer's was pretty advanced. She's non-communicative. We are really just beyond words."

As soon as they found out that Mustaine's mother-in-law was missing, an extensive search-and-rescue operation quickly commenced. The operation involved teams from four countries, the US Border Patrol and 300 volunteers, including some bloodhounds and cadaver dogs.

Sadly, despite the exhaustive efforts, Estabrook has not been found yet. "It's been a remarkable outpouring of volunteers," shared Mustaine. "They've gone out with ATVs, horses, motorcycles. We've had people offer to use search drones. But, there's been nothing. No scent. She just vanished."

At first, Dave Mustaine had asked the public to check local hospitals, senior centers, and shelters. He even advised the people to look right in their own backyards. "A lot of times, people that wander who have Alzheimer's are found very close to their homes," Mustaine said. 'They just take shelter underneath bushes."