For years Facebook users have long suspected that the social media site has been tracking their online movements. Their fears were not alleviated if they watched Mark Zuckerberg’s testimony on Capitol Hill on Tuesday.

The Facebook CEO was grilled by senators for hours about the company’s role in data privacy scandal that resulted in 87 million users having their data improperly accessed by a data analytics company.

Zuckerberg, who founded Facebook in 2004, was asked by Senator Roger Wicker (R-MS) about the company’s ability to track users after they leave the site.

“There have been reports that Facebook can track users’ internet browsing activity even after that user has logged off the Facebook platform. Can you confirm whether or not this is true?,” Wicker asked.

Zuckerberg wasn’t sure if he could give an accurate response and told the senator from Mississippi that someone from his company would follow up.

“You don’t know?,” said Wicker.

“I know that people use cookies on the internet and that you can probably coordinate activity between sessions,” Zuckerberg said. “We do that for a number of reasons, including security and including measuring ads to make sure that the ad experiences are the most effective, which of course people can opt out of. But I want to make sure I’m precise in my answer.”

Wicker also wanted to know how the social media platform informs users about the tracking.

The exchange marked one of the more head-scratching moments in Zuckerberg’s five-hour testimony before the Senate Judiciary Committee where he repeatedly told members of Congress that someone from his team would get back to them with specific answers.

Check out the exchange below.