what piece of information about Chris did Westerberg provide to the state troopers before they took him seriously?

As soon as he (Westerberg) got to Carthage, a dispirited Westerberg phoned the Alaska State Troopers to volunteer what he knew about McCandless. By that time, however, stories about the dead hiker, including excerpts from his diary, had been given prominent play in newspapers across the country. As a consequence the troopers were swamped with calls from people claiming to know the hiker’s identity, so they were even less receptive to Westerberg than they had been to Gallien. “The cop told me they’d had more than one hundred fifty calls from folks who thought Alex was their kid, their friend, their brother,” says Westerberg. “Well, by then I was kind of pissed at getting the runaround, so I told him, ‘Look, I’m not just another crank caller. I know who he is. He worked for me. I think I’ve even got his Social Security number around here somewhere.’” Westerberg pawed through the files at the grain elevator until he found two W-4 forms McCandless had filled out. Across the top of the first one, dating from McCandless’s initial visit to Carthage, in 1990, he had scrawled “exempt exempt exempt exempt” and given his name as Iris Fucyu. Address: “None of your damn business.” Social Security number: “I forget.”

But on the second form, dated March 30, 1992, two weeks before he left for Alaska, he’d signed his given name: “Chris J. McCandless.” And in the blank for Social Security number he’d put down, “228-31-6704.” Westerberg phoned Alaska again. This time the troopers took him seriously.