``Casey? You don't know me but I know you. If you want a picture get to the corner of Adams and Clark just as fast as you can. If you hurry you might beat the headquarters boys.''

Casey heard the click of the distant receiver before he could open his mouth, and it took him no more than three seconds to make his decision. For over the years he had received many such calls. Some of them came from people who identified themselves. Some telephoned because he had done them a favor in the past. Others because they expected some sort of reward for the information. A few passed along a tip for the simple reason that they liked him and wanted to give him a break. Only an occasional tip turned out to be a phony, and, like the police, Casey had made a point of running down all such suggestions and he did not hesitate this time.

He was in his car with his camera and equipment bag in less than two minutes, and it took him only three more to reach the corner, a block from Columbus Avenue. It was a district of small factories and loft buildings and occasional tenements, and he could see the police radio car as he rounded the corner and slammed on the brakes. He did not bother with his radio -- there would be time for that later -- but as he scrambled out on the pavement he saw the filling station and the public telephone booth and knew instantly how he had been summoned.

The police car had pulled up behind a small sedan, its headlights still on.