Kelly is among several government officials who have now said that a "wall" across the entire border is not necessarily realistic. Instead the barrier will likely be some combination of wall, fencing, electronic surveillance and personnel, according to officials.
What the "wall" will look like -- including how tall and thick it will be -- is yet to be determined, Kelly said. It is "unlikely that we will build a wall or physical barrier from sea to shining sea," he continued, adding that he is "committed" to constructing a physical barrier wherever it is recommended.
"I have a lot of elbow room," he said.
On Wednesday, Customs and Border Protection released its apprehension numbers for the month of March, which continue to show a sharp decline.
Apprehensions -- used as an indicator of illegal border crossings -- were down 63 percent on the Southwest border: from 33,316 in March 2016 to 12,193 this past March, according to CBP.
During Wednesday's hearing, Kelly said that the U.S. has "seen an amazing drop in the number of migrants coming out of Central America that are taking that terribly dangerous route" from their home countries.
Kelly added that the reduction "won't last unless we do something again to secure the border."
Later in the hearing, Kelly explored other reasons why he thinks that fewer people are crossing the southwest border into the U.S., which include improving economic conditions in migrants' home countries and rising prices charged by coyotes helping to smuggle people across the border.