Welp, turns out it was both the French and U.S. embassies:
"The investigations revealed that the suspects were intending to carry out terrorist bomb operations inside Egypt via suicide operations, penetrating the security cordon in front of the American and French embassies with a car bomb," MENA said, citing a source in the state security prosecutor's office.
MENA said the suspects had escaped from prison during the uprising that swept Hosni Mubarak from power in 2011.
FWIW, there's no way even a car bomb could penetrate the walls at the U.S. embassy. It's a goddamn fortress. It would basically just blow a hole in the wall itself and kill some Egyptian security guards and bystanders (unless they timed it for when all the shuttles are leaving; that's the scenario that really scares me but it would be tough to pull off with the randomized schedules). Even if they used the Taliban tactic of following up the bombing with gunmen, they wouldn't find more than 4-5 people in the courtyard (and again, mostly Egyptians). The buildings inside the perimeter are also all independently secured. Their remote controlled airplane idea was probably better, but would still likely be more of a shock value than high casualty rate.
No idea on the French embassy.
