Former U.N. special rapporteur Richard Falk, who investigated Israeli atrocities against Palestinians, said Canadian officials questioned him on "national security” grounds when he arrived in the country this week to attend a Gaza-related event.

Falk told Al Jazeera that he and his wife, legal scholar Hilal Elver, were questioned by authorities Thursday at Toronto Pearson International Airport.

"A security person came and said, ‘We’ve detained you both because we’re concerned that you pose a national security threat to Canada,’” Falk said Saturday in an interview from Ottawa.

"It was my first experience of this sort – ever – in my life," he added.

Falk and Elver, both U.S. citizens, were traveling to the Palestine Tribunal, which convened international experts to examine Canada’s role in Israel’s genocide in Gaza.

He said the couple was held for over four hours and questioned about their work on Israel, Gaza, and genocide more broadly.

Stating that nothing was "particularly aggressive" about the interrogation, he described it as "sort of random and disorganised."

However, Falk said he believes the questioning is part of a wider international effort to "punish those who endeavour to tell the truth about what is happening" around the world, including in Gaza.

"It suggests a climate of governmental insecurity, I think, to try to clamp down on dissident voices," the former U.N. rapporteur added.

For its part, the Canada Border Services Agency (CBSA), which oversees the country’s border crossings, told Al Jazeera that it cannot discuss individual cases due to privacy rules.

CBSA spokesperson Rebecca Purdy said assessing security risks and admissibility may involve primary and secondary inspections, which are routine "and should not be viewed as any indication of wrongdoing."

Canadian Sen. Yuen Pau Woo, who supports the Palestine Tribunal, said he was "appalled" that two international law and human rights experts were questioned in Canada over a potential national security risk.

"We know they were here to attend the Palestine Tribunal. We know they have been outspoken in documenting and publicising the horrors inflicted on Gaza by Israel, and advocating for justice," Woo added.