President-elect Donald Trump's transition team asked the Department of Homeland Security about an aerial surveillance program along the southern border that was scaled back by the Obama administration, Reuters is reporting.

The program, Operation Phalanx, deployed as many as 6,000 National Guard airmen to monitor for drug trafficking and illegal immigration during President George W. Bush’s administration.

The Trump team's inquiry was part of a wide-ranging request for documents and analysis looking into the viability of a border wall and barrier construction.

According to Reuters, the U.S. Customs and Border Protection identified more than 400 miles along the U.S.-Mexico border where new fencing could be erected.

About the same distance can be fenced off along the U.S.-Canada border, according to a document accessed exclusively by Reuters.

During his presidential campaign, Trump emphasized repeatedly that securing the border would be his first immigration priority.

Nearly 700 miles of fencing was put in place during President George W. Bush’s second term and the beginning of President Barack Obama’s first term -- but is not the type of solid wall that Trump has pledged to construct at Mexico’s expense.

The fence has miles-long gaps and gates built in to allow landowners access to their property on the south side of the fencing. Immigrants have been known to go over and around the fence.

The AP contributed to this report.