ABC’s George Stephanopoulos was in disbelief Sunday when Democratic presidential frontrunner Hillary Clinton did not know about a long-standing legislative proposal that would allow the families of 9/11 victims to sue state sponsors of terrorism in federal court.

Stephanopoulos, a former Bill Clinton aide who has donated thousands of dollars to the Clinton Foundation, explained how the Obama administration opposes a bill sponsored by Sen. Chuck Schumer (D., N.Y.) and supported by the 9/11 families, known as The Justice Against Sponsors of Terrorism Act, to let them take legal action against state sponsors of terror.

The administration argues the legislation would expose the United States and its officials to retaliation.

"So who’s right here, Senator Schumer or President Obama?" Stephanopoulos asked Clinton.

"I don’t really know about that George; I’ll have to look into it," Clinton responded. "Obviously, we’ve got to make anyone who participates in or supports terrorism pay a price, and we’ve also got to be aware of any consequences that might affect Americans, either military or civilian or our nation. So I’m not able to – "

"You don’t know about this issue? It’s been around for several years," Stephanopoulos interjected, with a look of disbelief on his face.

"Well, I know there’s been an issue about it for quite some time," Clinton said in response. "I don’t know about the specific legislation that you’re referring to. But obviously, I’ll look into it."

Stephanopoulos then asked for clarification if Clinton is unwilling to take a position on the bill at this time.

"I can’t, I haven’t studied it," Clinton said, shaking her head.

The former secretary of state then took a shot at her Democratic primary opponent, Sen. Bernie Sanders (I., Vt.), adding, "Unlike some people I actually do try to learn what’s at the core of any question before I offer an opinion because, you know, it’s not enough to say what’s wrong; I think you got a responsibility to say how you’re going to fix it."

The Clinton campaign has made a point in recent of weeks of saying that Sanders can describe the problems the country faces but cannot come up with real, practical solutions to address them.

The Justice Against Sponsors of Terrorism Act passed the Senate in late 2014.