U.S. President Barack Obama said he was treating the attack at the King Abdullah Training Centre - in which four Jordanians and one Lebanese citizen were wounded - very seriously and a full investigation was under way.
The gunman was a police captain, a senior Jordanian official told Reuters. There was no immediate word on the attacker's background or motive. But Jordan is a staunch U.S. ally in the Washington-led campaign against Islamic State militants who hold large areas of neighboring Syria and Iraq, a position Jordanian officials say leaves the kingdom vulnerable to jihadist attacks.
"This incident sadly does not come as a surprise as the threat of Islamist terrorism has only increased in the region in the last few years in the aftermath of Syria and Iraq. As much as pre-emptive measures have been taken, it is impossible to eradicate all risks," said a senior Jordanian official who requested anonymity, citing political sensitivities.
The U.S. Embassy in Amman said two American civilian security contractors and a South African contractor were shot to death, and the slain Jordanian was a translator according to the Jordanian government. It was "premature to speculate on motive at this point" with the investigation proceeding, it said.
A U.S. official said the two Americans were working for the U.S. State Department's International Narcotics and Law Enforcement Affairs Bureau training Palestinian security forces.
Obama told reporters at the White House: “The fact that someone dressed in military uniform carried out an attack at a training facility in (Jordan)..., we take this very seriously,
and we’ll be working closely with the Jordanians to determine exactly what happened."
The shooting spree took place on the 10th anniversary of al Qaeda suicide bombings that targeted three Amman luxury hotels and killed 57 people, the deadliest militant attack in Jordanian history.
EARLIER ATTACK PLOTS THWARTED
Security sources said several earlier militant plots to attack the King Abdullah training center had been foiled.
Jordanian government spokesman Mohammad Momani said the attacker was gunned down by Jordanian security forces inside the training center. He did not commit suicide as security sources had earlier reported.
The training facility was set up on the outskirts of the capital Amman after the 2003 U.S. invasion of Iraq to help rebuild the shattered country's post-war security forces and to train Palestinian Authority police officers.
Jordan hosts several hundred U.S. contractors who are part of a military program to bolster the kingdom's defenses, including the stationing of F16 fighter jets that use Jordanian airfields to hit Islamic State positions in neighboring Syria.
But Jordan's role in the war against Islamic State has raised disquiet among some Jordanians about instability at their borders. They fear that Amman's enhanced role in the campaign might provoke Islamist bloodshed in their country.
Jordanian authorities have since tightened security around sensitive government districts, increased surveillance of radical Islamists and jailed dozens on suspicion of plotting militant attacks on Israelis, Americans and other Westerners.
King Abdullah believes fervently that ultra hardline jihadists pose an existential threat to the kingdom.
Since the civil war erupted in Syria in 2011, hundreds of Jordanians have joined Sunni Muslim militant groups in the insurgency against President Bashar al-Assad, according to Islamists close to the subject.
But Jordan's Western-aligned monarchy is torn by conflicting interests over Syria. It has tried to steer a middle course between that of Gulf Arab allies who want Assad ousted at almost any cost and its own concerns - echoed by Washington - that a radical Islamist victory in Syria would install a worse threat.
That ambivalence means that while Jordan has hosted small-scale U.S. training of rebels and allowed modest quantities of Gulf-supplied arms to filter into Syria, it has ensured that its border has not become an easy conduit for guns and combatants.