Acting Defense Secretary Patrick Shanahan is making his second trip to the U.S.-Mexican border to highlight what President Donald Trump calls a national emergency, after freeing up $1.5 billion more in Pentagon money to support wall construction.

Shanahan was flying to the Texas border town of McAllen on Saturday with the acting chief of Homeland Security Department, Kevin McAleenan, for a trip that demonstrates Shanahan's attention to border security, a top Trump priority, amid questions from some in Congress about whether the border mission is an appropriate and wise use of military resources.

As a prelude to the trip, the White House on Thursday announced that Trump intends to nominate Shanahan as defense secretary, ending months of speculation about Pentagon leadership. He has served in an interim capacity since Jan. 1, an unprecedented period of uncertainty at the helm of the Pentagon.

Shanahan has supported the use of active-duty troops, in addition to the National Guard, to bolster Customs and Border Protection efforts to handle surging numbers of Central American migrants seeking to cross the border. But recently he has hinted at impatience with the lack of a long-term strategy for ensuring border security, which is the responsibility of DHS.

In congressional testimony May 1, Shanahan said he and Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, have been considering the question of how long the military will be needed at the border and how best it can support that need.

"The question he and I are trying to answer," Shanahan said, "is, how long will we be at the border." He added, "We really need to get back to our primary missions and continue to generate readiness" to undertake conventional military operations.

On May 3, Shanahan told reporters that the border crisis had developed more quickly than anyone had anticipated, putting extra pressure on DHS.

"I don't think anybody thought it would be this bad, the situation would deteriorate like it has, and that distress would be as high on those front-line (DHS) employees," he said.

This past week, Shanahan told Congress there are 4,364 military troops on the border, including active-duty and National Guard. They are erecting barriers, providing logistics and transportation service and other activities in support of CPB. The troops are prohibited from performing law enforcement duties.

Many Democrats, including Sen. Dick Durbin of Illinois, have questioned the use of active-duty troops on the border.

"The longer the Southwest border mission continues, the line of demarcation starts to blur in terms of where we're drawing a line saying this is not a military responsibility, this is law enforcement, immigration, internal security responsibility," Durbin said at a recent hearing.

On Friday, Shanahan announced he was transferring $1.5 billion from numerous defense projects, including $604 million originally intended for use in support of Afghan security forces, to a Pentagon counterdrug fund that will help finance construction of barriers on the U.S.-Mexico border. That is in addition to $1 billion the Pentagon transferred for wall construction in March.

The backdrop to Shanahan's trip is his pending nomination. Shanahan has served as the acting secretary since Jan. 1, when Trump elevated him from deputy secretary to replace Jim Mattis, who resigned in December.

The White House has never explained why it took Trump so long to decide to nominate Shanahan, a former Boeing Co. executive. Trump himself has said he likes to keep Cabinet members in an acting status because gives him more flexibility, although it also frustrates the Senate's efforts to exercise its constitutional role of providing advice and consent.

In March, the Defense Department's inspector general investigated accusations that Shanahan had shown favoritism toward Boeing during his time as deputy defense secretary, while disparaging Boeing competitors. The investigation appeared to stall his nomination, but the internal watchdog wrapped up the inquiry in April and cleared Shanahan of any wrongdoing.