A Wall Alone 'Will Not Do the Job,' Says Trump's DHS Chief Pick

President-elect Donald Trump's nominee for the secretary of the Department of Homeland Security, retired four-star Marine Gen. John Kelly, said during his Senate confirmation hearing today regarding a border wall that "a physical barrier in and of itself will not do the job. It has to be really a layered defense."

"If you were to build a wall from the Pacific to the Gulf of Mexico," you would still have to back that wall up with patrolling by human beings, sensors and observation devices, he said.

Kelly, the former commander of U.S. Southern Command, which oversees military activities in Central and South America, said that the defense of the southwest U.S. border starts about 1,500 miles south, and includes partnering with countries as far south as Peru to go after drug production and transport.

He also said that technology would also have to be a big part of border security -- unmanned aerial vehicles, sensors, etc.

The promise to build a wall was a cornerstone of Trump's campaign.

As DHS secretary, Kelly would face ongoing issues of dealing with issues of immigration, border security, domestic terrorism threats and cybersecurity. DHS is a notoriously large and complex department that was established after the 9/11 terror attacks in an effort to better coordinate within the federal government. Multiple independent agencies were moved under its domestic security umbrella.

DHS was at the bottom of the 2016 big agency list among the annual "best places to work" in the federal government, despite the first increase in its rating in years. Outgoing DHS Secretary Jeh Johnson made employee engagement one of his priorities during his tenure.

When asked about his views on the Obama administration immigration program, Deferred Action for Childhood Arrivals (DACA), Kelly said that the entire development of Trump's immigration policy is still "ongoing right now."

"I would guess" that this category of potential deportations "won't be the highest priority" in the Trump administration, Kelly said.

"I'll follow the law," he said when questioned on whether he would use "limited resources of law enforcement community" to deport DACA recipients.

Kelly told the senators that he doesn't agree with "registering people based on religion or ethnicity" or anything like that.

In an interview as a candidate, Trump indicated that he would support a database that tracks Muslims in the U.S., though Trump's team later said that Trump never advocated for "any registry or system that tracks individuals based on their religion."

Kelly said that the success he achieved during his time in Iraq was because he reached out to people "across the spectrum and society, all whom were Muslim." He said that "outreach to the community" was how he "won" during his time in the war-torn country.

Sen. John McCain, who was held captive and tortured in Vietnam, asked Kelly if he would agree to following Geneva Conventions rules on torture.

"Absolutely," said Kelly.

"I don't think we should ever come close to crossing a line that is beyond what we as Americans would expect to follow in terms of interrogation techniques," Kelly said.

Kelly enlisted in the Marine Corps in 1970. His first military deployment was to Guantanamo Bay in 1971 when he was just 20 years old. He graduated from the University of Massachusetts before returning to the Marine Corps and working his way up the ranks, with stints on aircraft carriers, in the nation's capital and at Camp Pendleton in California.

Unlike some of Trump's other Cabinet picks, Kelly has already been confirmed by the Senate five times for previous positions.

In addition to his experience leading troops overseas, he is known for his strong knowledge of border issues and the drug trade in South and Central America.

Kelly is also a Gold Star father. He lost his son, Marine 1st Lt. Robert Kelly, in an IED explosion in Afghanistan in 2010.

"I have a profound respect for the rule of law and will always strive to uphold it. I have never had a problem speaking truth to power, and I firmly believe that those in power deserve full candor and my honest assessment and recommendations," he said in opening remarks during today's confirmation hearing.

ABC News' Benjamin Siegel, Shushannah Walshe, Elizabeth McLaughlin and Alana Abramson contributed to this report.