yes, it must be a missile, because the explosion would have to be 250 km over the central US in order to be effective)
Didnt you ever watch James Bond: Golden Eye? The one with Peirce Brosnan and the driving the tank through Moscow and the EMP blasts etc etc?
Anyway, in case you didnt see it, the Russians had been experimenting with EMP blasts, and so put a bunch of nuclear warheads above what appeared to be communication satelites. Then they were detonated, the explosion didnt reach the ground but the EMP did and wiped out all sorts of tech in a remote part of Russia then it all became fantasy.
There was another movie... Space Cowboys i think it was called... the Russians built a 'communications satelite' filled it with 12 or so multi-warhead ICBM's and put it in space during the cold war. Then nuclear war didnt ensue and the Russian economy collapsed and everyone forgot about the satelite. Then, somehow, it was damaged and became active, so the russians went to the americans and asked for help repairing it. and thats how the americans found out.
Also, there have been a number of other books and things talking about EMP attacks. There was a weird micro series in Australia called Afterworld (im sure it aired in the US as well). it was basically 5 minute episodes that aired 3 times a day and you had to get all the info you could and try to guess the ending. what happened was one day all technology (on the planet, as far as we know) stops working, and like 90% of the earths population just diappears (im not sure if it was 90% of the people on earth or just the US we didnt find out before i stopped watching)
There is a really good book called Sunstorm by Stephen Baxter and Arthur C Clark. Its about a massive coronal ejection (solar flare) that shoots all kinds of nasty light, heat, radiation and whatever else right at us (Earth). It started with a massive EMP that shorted out most systems on the face of the earth (the face that was facing the sun at the time). then scientists learned it would happen again in 5 years but 100 times worse, and if they didnt do something to stop it the earth would be made uninhabitable. and the funny thing is that the flare was caused by an advanced race of aliens that believe that all energy in existence is finite, and will eventually run out at which point life and the universe will cease to be, so they are trying to stave off the inevitable by being super energy effecient, and policing other lifeforms to do the same. so when a race like humans come along and waste energy on weapons of war and space exploration and microwaveable popcorn, they try to eradicate us, using, funnily enough, the sun.
so this super energy conservative race tries to wipe out a reckless energy consuming species, by expelling enough energy to destroy a planet... ironic huh?
anyway, i wont go into the story more, because its too good a read to be ruined by explaining, but its DEFINITLY worth looking into.