As Iran retaliates, largest US military base in Middle East hit by ballistic missile, Qatar says

The Pentagon has identified four of the six American troops killed in the opening hours of the war with Iran.

Capt. Cody A. Khork, 35, of Winter Haven, Florida; Sgt. 1st Class Noah L. Tietjens, 42, of Bellevue, Nebraska; Sgt. 1st Class Nicole M. Amor, 39, of White Bear Lake, Minnesota; and Spc. Declan J. Coady, 20, of West Des Moines, Iowa, died Saturday in Kuwait from an Iranian drone attack.

All soldiers were assigned to the 103rd Sustainment Command, an Army Reserve unit based in Des Moines, Iowa.

-ABC News' Steven Beynon

The Senate will vote Wednesday on a Democratic-led Iran war powers resolution, according to a press release from Minority Leader Chuck Schumer and bill co-sponsors Sen. Tim Kaine and Sen. Adam Schiff.

The vote is set for 4 p.m.

The resolution would direct the removal of U.S. Armed Forces from hostilities within or against Iran that have not been authorized by Congress.

Because the resolution is privileged, it will only need 51 votes to advance. If the legislation gets the necessary votes, a second vote is still needed in the Senate on final passage.

-ABC News’ Allie Pecorin

Iranian clerics have issued fatwas calling on Muslims worldwide to avenge the killing of Ayatollah Ali Khamenei during the U.S.-Israeli attack on Iran, the Department of Homeland Security said in a "Critical Incident Note" obtained by ABC News.

The first fatwa was issued on Sunday by Grand Ayatollah Naser Makarem Shirazi and said avenging Khamenei's death "is the religious duty of all Muslims in the world," according to the document. The second fatwa was issued by Ayatolla Nouri Hamedani and said "avenging the blood of the martyred leader of the revolution is obligatory for all Muslims," according to the document.

Iranian religious leaders have previously issued fatwas against President Donald Trump and Israeli Prime Minister Benjamin Netanyahu.

The DHS document said the deadly shooting in Austin, Texas, on Sunday "probably preceded the issuance of the fatwas and the motive for the attack has not been confirmed." Authorities are continuing to explore whether there was a terroristic motivation.

-ABC News' Aaron Katersky

All six American service members were killed in the opening hours of the war against Iran by an Iranian drone at Shuaiba port in Kuwait, according to two defense officials.

The troops in Kuwait were killed in a command and control center, a building which is effectively a large trailer, according to the officials.

The center was encircled by 6-foot-tall concrete walls -- a staple in defending against rockets and mortars from the Iraq and Afghanistan war-era, but they do not protect against aerial attacks, according to the officials.

These centers are often ad hoc or semi-permanent locations for units to coordinate logistics and their forces.

-ABC News' Steven Beynon