CROSS CITY, Fla.  Warren Messner was 15 when he and some friends attacked a homeless man and left him for dead. Mr. Messner jumped on a log laid across the man’s ribs. He does not know why. He was high, does not remember much and wants to forget the rest.

Today Mr. Messner is a baby-faced 18-year-old serving 22 years for second-degree murder. He used to like skipping school and listening to rap music with friends. He imagined he eventually would help his father install flooring. Now he talks to his parents nearly every night from the maximum-security Cross City Correctional Institution.

“It was just a senseless crime.” he said, his eyes down, his shoulders slumped. “I wish it would have never happened. It made no sense. It was stupidity.”

Mr. Messner’s story is not unusual. Nationwide, violence against the homeless is soaring, and overwhelmingly the attackers are teenagers and young adults. In Florida the problem is so severe that the National Coalition for the Homeless is setting up speakers bureaus to address a culture that sees attacking the homeless as a sport. It is the first time the organization has singled out a particular state.