Not even they know who did.
The Sleepers had nothing to do with the magickal stifling of New York City in the ’50s. They have no problems letting everybody think they did, of course, but they’re pretty damn interested in finding out who the true culprits are.
At least, so the rumor goes. Like many rumors this one is far too vague to be of much use, and yet it sparks an interest — if they didn’t do it, there’s an awfully big fish out there that’s kept a secret for fifty years, possibly a bigger one than the Sleepers themselves. Since the action was in line with (known) Sleeper goals (and didn’t draw any undue attention from normals, just a lot of pissed-off adepts) they haven’t pursued it as much as other, more flagrant violators of policy. TNI, however, might be willing to expend a lot of resources following this rumor, just to help Alex Abel sleep a little better. So even if it turns out to be a total goose chase, this rumor can add a nice little twist.
A good way to start this rumor is to have the PC’s meet a survivor of an encounter with Sleeper agents who ask some very odd questions about Manhattan…