Set some rules for them to follow and give them some jobs to do. You'd be surprised at how resourceful they can be if told the rules and in how much time they will invest in helping you to solve their case. Victims do not like to be kept out of the loop. Set up some ground rules. You will call them when you have news and they may call you every 3 - 4 weeks for a status report.
Some departments have community liaisons.
A brief message on a voice mail can make the difference between a victim who feels supported and one who perceives you as a wall to climb. You might even find out about a long-forgotten detail or one that was overlooked in the preliminary interview. This victim will continue to live with this crime long after you have closed the file. They need to know you have tried your best and that the victim has not forgotten. There is no easy answer to identity theft.