Homeland Security Secretary Alejandro Mayorkas dodged questions from senators Tuesday about the criteria used to justify keeping the nation’s northern land border closed to fully vaccinated Canadians, hours after the U.S. renewed its border restrictions for another month.

Testifying on “threats to the homeland” before the Senate Homeland Security and Governmental Affairs Committee, Mayorkas would not directly respond to queries from Chairman Gary Peters, D-Mich., and Sen. Maggie Hassan, D-N.H., about the reasoning behind barring Canadians who are fully vaccinated against COVID-19 from driving to the U.S.

Mayorkas also signaled the fate of the northern border may be tied to that of the U.S.-Mexico border, where the federal government currently enforces strict asylum and entry restrictions amid record-high migration levels.

“We are taking it iteratively. We are looking at the situation, not only at the ports of entry at our northern border but also at our southern border,” he said.

The senators, who represent states along the northern border, noted that Canada has a higher vaccination rate than the U.S. and that the country recently lifted its own land border restriction on fully vaccinated American travelers.

Peters specifically questioned why the U.S. chose to lift travel restrictions on vaccinated visitors from more than two dozen countries beginning in November, an update announced Monday, but kept the Canadian border closure in place. Hassan also pressed Mayorkas on why the restrictions are limited to land, allowing Canadians to fly into the U.S. regardless of vaccination status.

“I do not understand the public health rationale here at all for closing the northern border to vehicular traffic when it is essentially open to air traffic,” Hassan said.

Peters asked Mayorkas to provide his office and the public with the “specific criteria the administration is using to justify ongoing restrictions for fully vaccinated Canadians at land ports of entry.”

Mayorkas said he “most certainly will” but did not provide any metrics at the hearing.

“We are looking primarily at the public health rationale, the fact that the arc of the delta variant is not yet where we need it to be,” he said.

The Homeland Security chief added that he was “mindful” of the effects the border closure has had on Canadian-American families and on border communities that rely economically on tourism. He said the administration is “reviewing both the public health and the family impact consequences of our decision on a daily basis.”

Hassan appeared dissatisfied with the responses, shooting back that Mayorkas’ comments were the “same responses we have gotten for weeks and months without anybody explaining to us the public health rationale” behind the border closure.

The federal government has restricted cross-border travel between the U.S. and Canada since March 2020, allowing only “essential travel” over the northern land border. The restrictions were previously mutual, until Canada opened its side of the land border to vaccinated Americans last month.

The U.S. has yet to reciprocate. Earlier Tuesday, the Biden administration posted a notice extending its land border restrictions with both Canada and Mexico until Oct. 21, at which point the government has the option to renew them.

Tuesday’s filing said that Mayorkas “has determined that the risk of continued transmission and spread of the virus associated with COVID-19 between the United States and Canada poses an ongoing ‘specific threat to human life or national interests.’”

While much of the focus in Congress has been on high levels of migration to the southwest border, lawmakers from both sides of the aisle have ramped up pressure on the Biden administration to reopen recreational travel with Canada.

On Friday, New Hampshire Sen. Jeanne Shaheen led a letter to the administration, signed by seven other Senate Democrats from northern border states, calling for fully vaccinated Canadians to be allowed to cross the land border.

“We appreciate the need to prioritize the health and safety of the American public through reasonable restrictions on international travel. However, we believe that fully vaccinated Canadians should be allowed to safely travel into the United States via land ports of entry,” they wrote.

Rep. Elise Stefanik, R-N.Y., and Sen. Steve Daines, R-Mont., also introduced bicameral legislation in June that would expand the types of travelers permitted to cross the U.S.-Canada border and require the Department of Homeland Security to share with Congress its plans to fully reopen travel on the northern border.