Detectives Paul Westlund and David Burns, of the Criminal Investigation Division, leave HUPD's 29 Garden St. headquarters for the Harvard Sailing Center near MIT, where a wallet was stolen a few days ago. Thanks to a surveillance camera in a store where a stolen credit card was used, the detectives have identified a suspect.

"When you see a crime like this, you can start narrowing down some suspects," Westlund says. "Not everyone does this."

HUPD has five full-time detectives, including Westlund and Burns, in the division. The detectives don't do daily patrols. Instead, they take a longer view, following up on thefts and robberies and hunting for patterns.

A photo array does not positively identify a suspect in the Sailing Center theft, but the detectives remain hopeful as they head back to the station.

2:50 p.m.--Westlund happens upon a minor motor vehicle accident on JFK Drive and offers help.

2:57 p.m.--The report they've been waiting for all day finally comes in: someone matching the description of the lab larcenies has been spotted near Mallinckrodt Chemistry Lab. The report is about 20 minutes old, the dispatcher reports; a few minutes later, another officer radios that the man has boarded an MBTA bus.

Officers take off in pursuit, radioing for permission to stop the bus. Permission is granted after the units pass over the Somerville line. Officer Scott Green flashes his lights to get the bus driver's attention.

Green pulls the man off the bus and it drives off. Westlund and Burns interview him. But it's a false alarm--the wrong man, they decide.

Maybe next time, everyone hopes.

"The thing about Harvard is that because it's an open campus, you can get in and out pretty easily," Westlund sighs as he drives back towards 29 Garden.

3:37 p.m.-- He's almost to the garage when the radio crackles with a fresh report. Again a suspicious person is reported in the Mallinckrodt lab area. The lights flash and sirens wail anew. The description is the same as earlier. This time, the report is about five minutes old. The voice on the radio tingles with excitement.

"I've got a witness who places him in the building in the last couple of minutes!" an officer radios triumphantly.

Units begin circling, drawing the noose tighter. Burns hops out of the car as Westlund circles a nearby parking lot, just waiting for a sighting.

Officer Julie Bates radios that she's located the suspect and is in foot pursuit--words that throw the other units into high gear. Westlund races up and down Oxford St. in the unmarked cruiser, shouting "Where are you? Where are you?"

Suddenly, the chase crosses the road--a flash of gray and blue followed by the blur of Bates's black uniform.