Gil Kerlikowske, the U.S. Customs and Border Protection commissioner, said that the number of children picked up since October now stands at 57,000, up from 52,000 in mid-June, and more than double what it was at the same time last year.
The situation, Kerlikowske said, "is difficult and distressing on a lot of levels."
Their testimony came a day after President Barack Obama appealed to Congress to give him $3.7 billion in emergency spending to deal with the crisis, which has become a political test for his administration as it sparks fierce debate on Capitol Hill and beyond.
The president was in Texas Wednesday for a fundraising trip, but he resisted calls to visit the border. Instead Obama planned to meet in Dallas with faith leaders and Texas officials, including Republican Gov. Rick Perry.
The crisis gave Republicans a fresh opportunity to question the Obama administration's competence even as it complicated the debate over the nation's fractured immigration laws.
Republicans at Wednesday's hearing slammed Obama administration policies, blaming his efforts to relax some deportation rules for contributing to rumors circulating in Central America that once here, migrant kids would be allowed to stay.
"We're trying to stop human trafficking. Are we actually increasing it?" asked Sen. Ron Johnson, R-Wis.
Sen. John McCain, R-Ariz., noted that a high percentage of the children fleeing El Salvador, Honduras and Guatemala are able to succeed in their goal of staying in America because they're issued notices to appear at court hearings in the distant future, and many never do.
"Your odds are pretty good," McCain said. "There is ample incentive for them to come to this country."
The Department of Justice announced Wednesday that it's changing its priorities in order to respond to the crisis. Juan Osuna, director of the executive office of immigration review, testified that deportation cases involving unaccompanied child immigrants and parents traveling with children would be moved to the top of court dockets. That means lower priority cases will take even longer to wend through a court system where there's currently a backlog of more than 360,000 pending deportation cases.
Obama's emergency spending request would add more judges to the court system, but elsewhere on Capitol Hill Wednesday Republicans raised questions about it. House Speaker John Boehner was noncommittal about whether the House would vote on it.
"If we don't secure the border, nothing's going to change. And if you look at the president's request, it's all more about continuing to deal with the problem," Boehner told reporters. "We've got to do something about sealing the border and ending this problem so that we can begin to move on with the bigger question of immigration reform."
Obama's decision to skip a border visit was providing more fodder for the Republicans and the handful of Democrats who say the president hasn't responded quickly and forcefully enough to the mounting crisis.
Perry, a possible GOP presidential candidate in 2016, has been scathing in his criticism of Obama, saying the White House has failed to respond to his repeated warnings about a flood of minors at the border. But Obama spokesman Josh Earnest said the White House was not worried about the optics of the president traveling to Texas without visiting the border.
Associated Press writers Alicia A. Caldwell, Julie Pace and Eric Tucker contributed to this report.