Kristi Cornwell missing case police sketch

I wonder why the grandmother worried her grandson might be involved in Kristi Cornwell's disappearance wrote a letter to the police and didn't provide any contact info. Now that the police have had to go public with the information to find out who wrote the letter, it seems to me that the life of the letter writer could be in danger. I must be missing something here.

MyFox Atlanta

GBI Gets New Lead in Cornwell Case
MyFox Atlanta
Investigators said that someone saw a police sketch and contacted investigators in Cherokee County, North Carolina. "We give it a high degree of credibility ...