A committee in Canada's Parliament voted unanimously Tuesday evening to take the rare step of issuing a subpoena demanding appearances from Facebook CEO Mark Zuckerberg and COO Sheryl Sandberg. Bob Zimmer, chair of Canada's Standing Committee on Access to Information, Privacy and Ethics, said in a phone call with CBS News that the subpoena can only be enforced if the Facebook executives step foot on Canadian soil.

"We're hoping this shows them we're serious. There are representatives from 10 countries now who want to hear from them," Zimmer said.

The move comes after Zuckerberg ignored repeated requests to address both a Canadian committee investigating disinformation and election meddling on the social media platform, as well as an "International Grand Committee" of legislative investigators from 10 nations set to gather in Ottawa May 28.

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Asked about the subpoena, a Facebook spokesperson said in a statement to CBS news that a company representative might attend the hearing.

"Over the last year, we have participated in numerous meetings and hearings with government bodies around the world. We will work with the Committee to ensure Facebook is appropriately represented at the May 28 meeting," the Facebook spokesperson said.

Initially composed of legislators representing eight nations, the International Grand Committee has since grown to 10 with the additions of members representing Brazil, Latvia and Singapore.

If Zuckerberg and Sandberg fail to respond to the subpoena or decline to appear, Zimmer said the committee will ask Parliament to take an extraordinarily unusual measure.

"The next escalation would be to submit to the full Parliament a motion to hold them in contempt," Zimmer said.

If passed, it would be just the sixth time in Canada's history that the House of Commons approved a contempt of Parliament citation.

Canada is now the second country to which Zuckerberg cannot travel without expecting to be immediately summoned to appear before government officials. After the International Grand Committee met for the first time in London last year — a meeting Zuckerberg declined to attend — the head of a British committee investigating disinformation told CBS News that if Zuckerberg ever steps foot in England he can expect a visit from authorities.

"If Mark Zuckerberg came to the U.K., we would serve a summons on him and if he refused to accept that summons then we could start contempt proceedings against him," Damian Collins, the chair of the Digital, Culture, Media and Sport Committee in the U.K. Parliament, said in a February interview.

The International Grand Committee first banded together Nov. 27 after they were stymied in their efforts to question leading tech executives for their separate disinformation investigations. When Zuckerberg didn't show up at that November meeting, he was represented by an empty chair.

Zimmer said the May 28 hearing will include "experts on disinformation from around the world." Confirmed attendees so far include Ellen Weintraub, Chair of the United States Federal Election Commission, Jim Balsillie, founder of Blackberry and Jason Kint, CEO of Digital Content Next, a trade organization that represents online publishers.