[Robert A Cook PE]

Could have gotten out through tunnel.

But, think of how many tunnels and safe houses and bunkers all connected with tunnels you can practically build in a limited time secretly.

Just isn't a lot of room for the excavations and the dup-up dirt to go if you're in a residential neighborhood, and you'd need loooooong tunnels to get back to the government site bldg "exit" ... Unless nearby.

Also, a blast at one end of tunnel (big blast, blowing up deep as these were) blows down the tunnel and kills people, collapses its ceiling, or burns/buries the bodies in the tunnel.

[Fledermaus]

Also, a blast at one end of tunnel (big blast, blowing up deep as these were) blows down the tunnel and kills people, collapses its ceiling, or burns/buries the bodies in the tunnel.

Or a one-ton nuke. Or a microwave bomb.