Sgt. Ron Brown's bloodhound sometimes becomes so intent on tracking down a missing person or criminal that he runs into things right in front of his wrinkled nose.

"They concentrate single-mindedly. Mine runs into trees - BOINGG," Brown said, snapping his head back. "They sometimes do crazy things like stop and smell a butterfly, but you just let them do it. They're the best man-trailing dogs in the world."Brown, who works with Maryland's Allegany County Sheriff's Department, was one of 80 police-dog handlers from 16 states who were in western Maryland on Monday to practice and refine their tracking skills deep in the woods and along city streets.

The handlers virtually played hide-and-seek with some of the 60 dogs attending the National Police Bloodhound Association's annual one-week training school at a cluster of cabins in Garrett County.

Instructors walked zigzag trails, and the bloodhounds, who can sniff out a person's scent for miles, would track their whereabouts.

Sometimes the instructor would hop in a car at the end of the trail so the handlers could learn how their dog reacts when it no longer can find the scent.

"The handler has to be close enough to his dog to read his dog. You have to be able to know when he's working," said Ralph "Jim" Suffolk Jr., a retired New York State trooper who handled bloodhounds for 20 years.

Bloodhounds, known for their red-rimmed, sad eyes, drooling mouths and wrinkled foreheads, can begin tracking when they are just weeks old. Some dogs work more than 12 years tracking escapees, criminals and lost children and elderly persons, including those with Alz-heimer's disease.

The dogs often begin their work by sniffing clothing worn by the person being tracked. The bloodhounds can also pick up scents from car and bicycle seats, key rings or wallets - anything a person has touched.

Retired Connecticut State Police Trooper Andrew J. Rebmann, who once scented a dog with a used diaper, said body scent is different than body odor. The human body continually sheds millions of microscopic cells. It is believed that bacteria interact with the cells and produce a gas. Each gaseous odor is unique, like a fingerprint.