The U.S.-led coalition confirmed in a statement that 12 others were wounded when 18 107-mm Katyusha rockets impacted the large training facility at Camp Taji -- which houses U.S. troops -- at 7:35 p.m., local time in Iraq.
Following the attack, Iraqi Security Forces found a rocket-rigged truck a few miles from Camp Taji.
While the U.S. is not confirming who it believes is behind the attack, Iranian-backed militia groups have been found responsible for similar attacks on bases that house U.S. forces. And the Trump administration has warned that another Iranian attack that killed Americans would warrant a response.
Testifying on Capitol Hill on Thursday, the commander of U.S. troops in the Middle East said the attack on Camp Taji was "most likely Shia militia groups."
"I will note that the Iranian proxy group Kata'ib Hezbollah (KH) is the only group known to have previously conducted an indirect fire attack of this scale against U.S. and Coalition forces in Iraq," Gen. Kenneth McKenzie, the head of U.S. Central Command, told the Senate Armed Services Committee.
KH, which is backed by Iran, was found responsible for the December attack on a base in northern Iraq that killed an American contractor. That led the U.S. to take military retaliation against the group, carrying out airstrikes on several locations in Iraq and Syria.
Tensions continued to escalate, as demonstrators attacked the U.S. Embassy in Baghdad. Then on Jan. 2, the U.S. military conducted a strike on Qassem Soleimani, killing the top Iranian general as he left Baghdad International Airport. The U.S. said he was plotting additional attacks against U.S. personnel and interests in the region. But in a show of force, the Iranians launched more than a dozen ballistic missiles at bases in Iraq where U.S. troops were present, causing traumatic brain injuries to more than 100 American service members.
Speaking about the death on Soleimani in January, Pompeo said, "Judging from the type and intensity of the strike, the regime certainly must now understand what we will do if they ever again pose risk to American lives. If Iran escalates, we will end it on our terms."
"I would believe a red line for the United States is gonna be the death of U.S. service members or those of our partners and allies. So that’s a red line," McKenzie said on Thursday.
Pompeo and others in the administration have claimed the strike on Soleimani re-established deterrence with Iran. But Pompeo has acknowledged "it's not everlasting." And McKenzie on Thursday expressed a more nuanced view, saying that while deterrence has been established regarding direct attacks between the U.S. and Iran the same cannot be said for Iran's use of proxies.
ABC News contributor Mick Mulroy, former Deputy Assistant Secretary of Defense for the Middle East, said that if the Camp Taji attack was determined to be supported by Iran the president would likely be presented with several courses of action to respond.
"If this action was supported or directed by Iran, it is incredibly provocative and could not have come at a worst time as it risks escalation of a regional conflict at a time of an international health and now potential economic crisis," Mulroy said.
There have now been four American troops killed in Iraq this year alone.
Over the weekend, two U.S. Marine Raiders, Gunnery Sgt. Diego Pongo and Capt. Moises Navas, were killed during an anti-ISIS mission in northern Iraq.
ABC News' Conor Finnegan contributed to this report.