Border enforcement veterans told lawmakers Tuesday that fencing along the U.S.-Mexico border has been "absolutely critical" in reducing violence, drug-smuggling and illegal entry -- in testimony boosting President Trump's call for a Texas-to-California wall.

Ronald Colburn, former Border Patrol deputy chief, and David Aguilar, former acting commissioner of Customs and Border Protection, testified to the Senate Homeland Security Committee about their experiences in the Bush administration when the first fencing was implemented.

Colburn also served as chief of the Yuma Border Patrol Sector, a stretch spanning southwestern Arizona and southeastern California, and was part of Operation Jump Start, which began in 2006 with the construction of a border fence.

Before the fence, the sector recorded 2,706 drive-throughs where illegal immigrants crossed the "unfettered" border with "contraband of drugs and people," Colburn said. Of those, Yuma Border Patrol captured just 13.

“The rest all got away, with no idea what, or who, they brought in,” Colburn said. “Yuma had become the most dangerous part of the border.”

Colburn also catalogued more than 200 attacks by “border bandits” — gangs that would “prey on their own,” sexually assaulting, robbing and murdering Mexican migrants.

But once the fence was installed in 2006, violent bandit attacks went from 200 to zero, he said. The implementation of the fencing barrier also cut the number of so-called drive-throughs from 2,706 to six.

“Yuma became the ‘proof of concept’ that America can protect and control its border when the proper mix of resources are placed almost instantaneously,” Colburn said. “By 2008, Yuma Sector arrests of illicit border crossers and traffickers had dwindled from over 138,000 to 8,363.”

Colburn, who has long supported the idea of physical barriers at the border, called fences critical, saying, “It marked our border; our line in the sand.”

Aguilar agreed that physical barriers at the border are an “integral part” of an enforcement system, but acknowledged that “challenges remain.” He cited environmental considerations; eminent domain; and tribal autonomy—the Tohono O’odham nation occupies 75 miles of the U.S.-Mexico border.

“The noted issues will have to be taken into consideration, but it is important to note that there is nothing more destructive to environmentally sensitive land and communities than the uncontrolled illegal flow of people, vehicles, smugglers, and criminal organizations,” Aguilar said. “The placement of fences and deterrent infrastructure in previously uncontrolled parts of the border have actually allowed for the rejuvenation of areas that had previously been devastated due to heavy illegal pedestrian and vehicular traffic.”

In 2015, DHS installed 353 miles of primary pedestrian fencing and 300 miles of vehicle fencing along the U.S.-Mexico border, which Aguilar said improved detection and surveillance capabilities, dropping migrant apprehensions from 1.7 million in fiscal 2000, to 408,870 in fiscal 2016.

“We have done much to secure the border, but there is much more to do,” Aguilar said.

DHS Secretary John Kelly will testify before the Senate Homeland Security Committee on Wednesday, just one day after the deadline for companies to bid to design and create President Trump’s border wall.

The plan continues to face intense Democratic criticism, as lawmakers warn the costly project would be an unfair burden on taxpayers.

"I don't think there's anyone in this Senate who doesn't want our border to be secure," Ranking Member Sen. Claire McCaskill, D-Mo., said Tuesday. "But the wall that President Trump has promised could cost nearly $70 billion -- that's $200 for every man, woman and child in the United States."

But the Republican chairman of the committee backed the call for more fencing along the border.

“We need a layered approach to border security, one that includes technology, manpower, a commitment to the rule of law, and the elimination of incentives for illegal immigration,” Chairman Ron Johnson, R-Wis., said. “Fencing does work, and we need more of it.”