Sort a like that Clancy novel, he wants
to bring the virus to America.
Contagion is probably the most likely method if they wish to kill a lot of us, but Ebola might not be the right one. Typically you will see symptoms within 10 days - longest observed incubation was 21 days. Of course a genetic variant may have been developed that takes longer, giving it ample time to to spread before word gets out that there is an outbreak. Granted, you could get a 12 Monkeys scenario where someone deliberately contracts the disease and then takes a 10-21 day trip through as many airports and destinations as possible. Even so, you can only really spread it by direct contact. An airborne contagion will spread far more quickly.