Transcript for Trump order putting military on US-Mexico border signed: White House

deployment begins immediately. Reporter: The homeld security secretary could not say how many troops or how much it would cost. As for what they'll do, U.S. Troops cannot do law enforcement functions such as arresting illegal border crossers. This would be a support role, such as helping with surveillance. It wi be strong. It will be as many as is needed to fill the gaps that we have today. That's what I can tell you. So, we'll go through and make sure. Reporter: Just last week, the president also suggested he would use military funds to build his border wall. Does the president have the authority to use money that congress appropriated the department of defense to build the border wall? What he meant washere are some lands that the apartment of defense owns right on the border that are actually areas where we see elicit activity. We're looking into options for the military to build wall on litary installations on the border. Eporter: But Pentagon officials tell ABC news the U.S. Military has no land on the Mexican border. The closest is the a force's Goldwater practice range in Arizona, which is not actually the border. Meanwhile, regarding T special counsel investigation, ABC news has confirmed president trump has been informed by Robert Mueller's team that he is not, at this point, a criminal target of the investigation. But he is a subject of the investigation, and therefore, Mueller wants to interview him. And what was his reaction to learning he is not a subject or that he not a target of the target of the special investigation, although he is a bject? We know what we did and what we didn't do, so none of this comes as much of a surprise. All right, Jonathan Karl joins us now. The president has said he's eager to talk to the special counsel, but that's something that's divided his legal team. Jon, where does that stand tonight? Reporter: Well, some of his legal advisers have told him point blank it would be a terrible idea to talk to Robert Mueller. But as you point out, the president himself has said repeatedly that he is willing, even eager to talk to Mueller and would do so UND oath. The white house has said nothg, Tom, to suggest that that has changed.

This transcript has been automatically generated and may not be 100% accurate.