“You’re right,” she said. “It’s not a tire track. It’s grease.”

She straightened and looked at the road. She saw no bloody tire marks, no auto debris. No pieces of glass or plastic that would have shattered on impact with a human body.

For a moment, no one spoke. They just looked at one another, as the only possible explanation suddenly clicked into place. As if to confirm the theory, a jet roared overhead. Rizzoli squinted upward, to see a 747 glide past, on its landing approach to Logan International Airport, five miles to the northeast.

“Oh, Jesus,” said Frost, shading his eyes against the sun. “What a way to go. Please tell me he was already dead when he fell.”

“There’s a good chance of it,” said Tierney. “I would guess his body slipped out as the wheels came down, on landing approach. That’s assuming it was an inbound flight.”

“Well, yeah,” said Rizzoli. “How many stowaways are trying to get out of the country?” She looked at the dead man’s olive complexion. “So he’s coming in on a plane, say, from South America-”

“It would’ve been flying at an altitude of at least thirty thousand feet,” said Tierney. “Wheel wells aren’t pressurized. A stowaway would be dealing with rapid decompression. Frostbite. Even in high summer, the temperatures at those altitudes are freezing. A few hours under those conditions, he’d be hypothermic and unconscious from lack of oxygen. Or already crushed when the landing gear retracted on takeoff. A prolonged ride in the wheel well would probably finish him off.”

Rizzoli’s pager cut into the lecture. And a lecture it would surely turn into; Dr. Tierney was just beginning to hit his professorial stride. She glanced at the number on her beeper but did not recognize it. A Newton prefix. She reached for her cell phone and dialed.