Free Among the Enemy by Margaret Peterson Haddix

cabin as soon as possible.
But when they got to Population Police headquarters— a monstrous building surrounded by a towering stone fence—he was chilled by the sight of guards everywhere. Hordes of them stood by the fence, by the gate, by every door and window. After Tiddy drove in past a guardhouse, a line of guards closed in across the road as quickly as a prison door slamming shut.
Oh, Percy, Matthias thought, as if his friend could really hear him, all those miles away, even you couldn't escape from this place.

Chapter Seventeen
Tiddy parked the car at a reckless angle and hollered over to one of the guards by the front door, "Hey, fill out the paperwork on this car for me, will you? We requi^ sitioned it from the enemy. It didn't come with a key."
Some of the guards chuckled, and Matthias heard one of them snort, "That's Tiddy for you."
"Come on, kid. You stick with me," Tiddy said to Matthias as they got out of the car.
"You requisition the kid, too?" one of the guards teased.
"Nope. He saved my life," Tiddy said, bounding up to the door.
"Yeah, sure," the guards muttered. "Can't wait to hear that story."
    Matthias followed Tiddy into the imposing headquarters building and through a maze of halls and stairs the way he had once followed Samuel through the reeking, traslvstrewn streets of his city. He didn't feel like he had much choice. It seemed like he'd cast his fate with Tiddy's when he'd shoved the officer into Mrs. Talbot's car. And, strangely, staying close to Tiddy helped staunch the fear pounding in his head: This is Population Police headquarters! Everyone here is evil!
    Then a guard standing before an interior door in a grand hallway planted himself firmly in front of Matthias, blocking his way.
"Officer Tidwell!" the guard chided, cutting his eyes dis' dainfully toward Matthias. "Surely you understand that it wouldn't be proper—"
"Proper?" Tiddy looked from the guard to Matthias. "Oh, relax. This is our newest Population Police member. He's been too busy perpetrating acts of heroism to get his uniform yet."
"Still, to go in there—," the guard persisted.
"Oh, very well," Tiddy said. "Send for a uniform. Size extra small."
Tiddy waited with Matthias until someone came with a uniform. It was gray, not black like Tiddy's. And it was at least three sizes too big.
'That's the best they can do down in the uniform room?" Tiddy asked, glancing at the sizing tag. "Oh, well. Just put it on over your clothes. You could use the extra bulk."
Matthias pulled the pants over his pajama bottoms. He had to make an extra hole in the belt to get them to stay up. He stuck his arm into the first sleeve. His sweater bunched up and rubbed uncomfortably.
    Oh no, he thought. I was planning to give Alia my sweater, way back before Percy was shot. ... In his rush to find shelter, tend his friends' wounds, and go for help, he'd totally forgotten. What if the man in the tree didn't take Alia someplace warm? What if she freezes to death because of me?
    Matthias's knees threatened to give out at this horrible thought. He sagged back against the doorway.
    Tiddy gently pulled the uniform shirt the rest of way around Matthias's body. He pushed Matthias's other arm through the other sleeve.
    "You don't have to button it," Tiddy said. "Come on, it's time to see the commander."
    Numbly, Matthias followed Tiddy past the guard and through a doorway that seemed, just by itself, to be taller and wider than most normal houses. The room beyond was so vast and awe-inspiring that Matthias came to a dead halt. A row of ornate chairs led up to a wooden desk that seemed as big as a car. The windows—all ten of them—stretched from the floor to the ceiling and were studded with colored glass.
And between the windows were black banners, just as tall and even wider—banners showing children dying, Population Police officers cheering.
It's like a cathedral, Matthias thought dizzily. A cathedral where people worship evil.
"Sir!" Tiddy was saying,