Senate Democrats and Republicans on Tuesday challenged the Obama administration to explicitly spell out its justification for using drones for targeted killings amid growing concerns about unchecked powers of the presidency and Americans’ civil liberties.

“Even as President Obama commands a military with the most sophisticated weapons known to man, including the weaponized drones used in targeted killing operations, his authority is still grounded in words written more than 200 years ago,” Sen. Dick Durbin, D-Ill., said at the start of a Senate hearing on the use of drones.

The Obama administration has successfully used drones in the war on terror and argued that the president’s authority stems from his constitutional power to protect the United States from imminent attack. The administration also has cited the 2001 Authorization for Use of Military Force, which says the commander in chief has the authority for strikes against al-Qaida and its affiliates.

Obama has used the law’s authority to target terrorists with fatal drone strikes, including Americans overseas.

The president has promised to explain his policy, but members of Congress argue that he has been less than forthcoming. The issue has created unique alliances on Capitol Hill with liberals joining forces libertarian-leaning Republicans.

Sen. Ted Cruz, R-Texas, told the Senate hearing that drones are technology, but the “real scope of this hearing and of the concern is on the scope of federal power.”

Both Durbin and Cruz expressed frustration with the administration, which declined to send a witness to appear before the Senate Judiciary subcommittee.

The hearing with retired military officers and outside experts comes a month after Kentucky Republican Sen. Rand Paul waged a nearly 13-hour filibuster of CIA Director John Brennan’s nomination over whether the president has the authority to use a drone to kill a U.S. citizen on American soil if the citizen is not engaged in combat.

Attorney General Eric Holder said the president does not have that authority.

(AP)