Lawmakers on Wednesday urged President Obama to release more information about his unprecedented use of drones to track and kill suspected terrorists, seizing on the administration's claim that Obama can even order the deaths of Americans on U.S. soil.

Both Republicans and Democrats grilled Attorney General Eric Holder on Capitol Hill, focusing on his insistence that Obama could target Americans domestically with armed drones in an "extraordinary circumstance" such as the Sept. 11 terrorist attacks.

"If an individual is sitting quietly at a cafe in the United States, in your legal judgment, does the Constitution allow a U.S. citizen on U.S. soil to be killed by a drone?" Sen. Ted Cruz, R-Texas, asked Holder.

"I would not think that would be an appropriate use of any kind of lethal force," Holder responded.

But the public remains largely in the dark about the rules for killings mostly ordered from thousands of miles away, and some members of Congress are trying to pressure Obama to outline a coherent blueprint for using the weapons.

Holder vowed on Wednesday that Obama would soon speak publicly on the issue and explain why it is necessary to continue the shadowy drone war. The administration has handed over some documents to congressional intelligence committees but that information, as well as other documents not released, remains classified.

"I believe the administration is really going to have to come to terms with this," said Sen. Dianne Feinstein, D-Calif., who called for greater transparency.

Dissatisfied with Obama's answers on drones, Sen. Rand Paul, R-Ky., announced a filibuster of John Brennan, architect of the drone program and the president's nominee to head the CIA. Paul accused the president of embracing the very principles he denounced during his first presidential campaign.

"Barack Obama, in 2007, would be down here with me arguing against this," Paul said from the Senate floor. "It amazes me and disappoints me how much he's changed."

Hours later, Democratic Sen. Ron Wyden of Oregon announced he would join the filibuster, making it a bipartisan effort. Wyden joined Paul in expressing concerns about the administration's drone operations.

The sentiment behind the filibuster was echoed by human rights organizations, which ripped the president for not shining a light on the controversial program.

"President Obama must do more to prove that his administration is serious about human rights," said Zeke Johnson of Amnesty International. "To that end, more public hearings on drones are needed, with survivors of drone strikes, independent experts on international human rights and humanitarian law and administration officials."

New revelations show that the administration is expanding the drone program on U.S. soil. According to documents obtained by the Electronic Privacy Information Center and CNET, the Department of Homeland Security is equipping Predator drones to detect whether Americans are carrying guns -- and the technology can track targets using their cellphone signals.

Still, Obama's use of drones remains popular with the American public, with polls repeatedly showing voters' support for the newfound method for rooting out suspected terrorists.

Even some Republican lawmakers came to Obama's defense on Wednesday.

"I want to applaud your effort with the drone program. I think it has really helped us in Afghanistan and Pakistan," Sen. Lindsey Graham, R-S.C., told Holder. "I just think it's a tactical tool that this president should be using and he's using it responsibly."