U.S. Homeland Security Secretary John Kelly will wrap up a two-day tour of the nation's border with Mexico on Friday as plans take shape to build a wall along the 2,000-mile divide between the two countries.
Kelly has told lawmakers that he would like to see wall construction well underway within two years, but he held open the possibility that it wouldn't extend to areas where there are natural physical barriers.
Fences already cover about 700 miles of the border.
Kelly was scheduled to tour one of the most fortified stretches of the border separating San Diego and Tijuana, Mexico. The two cities in the border's largest metropolitan area are separated by a double fence, much of it topped with razor wire.
San Diego is often cited as an example of how walls can slow illegal crossings, but critics say the structures only forced people to more dangerous areas where many have died in extreme heat.
The San Diego-Tijuana area of about 5 million people has the nation's busiest border crossing, where tens of thousands of motorists and pedestrians enter the U.S. every day. It's also one of the busiest crossings for cargo.
On Thursday, Kelly toured southern Arizona — the busiest corridor for illegal crossings from 1998 to 2013.
Southern Texas is now the most preferred route as large numbers of Central American families and children make their way to the U.S.
San Diego was the busiest route for illegal crossings until the late 1990s, when a surge of agents helped push crossers toward the remote mountains and deserts in Arizona.
Kelly was visiting the border in Arizona and California for the first time since he became secretary last month. Last week he toured the border in southern Texas.
Kelly, a retired four-star Marine general, rose to run the U.S. Southern Command, responsible for U.S. military activities in 31 countries in Central and South America and the Caribbean.
Kelly told a House panel on Tuesday that Trump's immigration and travel ban made "an awful lot of sense" but probably should have been delayed at least long enough to brief Congress about it.
On Thursday, a federal appeals court panel refused to reinstate the ban on travelers from seven predominantly Muslim nations. The three judges unanimously rejected the administration's claim of presidential authority.
This story has been updated.