Yes, that's one answer.
Or LE was tipped that the date for the plot would be today or tomorrow.
3 minutes ago, Fox radio news.
Sniper on rooftop, shooting at train. In New Jersey.
I got impression, it was a Metro type train, as they have shut it down.
Yes. LE must have known it was going down today. Or maybe those high tech bomb detection devices that are now placed outside major tunnels picked it up.