A Jordanian police officer shot and killed two Americans on Monday at a US-funded facility for training Iraqi and Palestinian security forces, before committing suicide, a security source said.
The Jordanian went on a shooting spree at the facility in Muaqar on the eastern outskirts of the capital Amman, the source, who requested anonymity, said.
Another security source said there were unconfirmed reports that at least three other US service personnel were injured.
Jordan is a staunch US ally and part of the Washington-led coalition that is trying to defeat Islamic State in Syria and Iraq, both of which border Jordan.
The country hosts several hundred US trainers who are part of a military program to bolster the kingdom's defense, including the stationing of F16 fighter jets that use Jordanian airfields to hit Islamic State positions in Syria.
But the kingdom's role in the war against Islamic State has caused disquiet among some Jordanians worried about instability at their borders and fearing that a stepped-up role in the campaign might lead to Islamist attacks in their country.
King Abdullah believes fervently that ultra hardline jihadists pose an existentialist threat to the kingdom.