AUSTIN, TX - APRIL 06: Texas National Guard helicopters traveling to the Texas-Mexico border prepare to fly on April 6, 2018 in Austin, Texas. Brigadier General Tracy Norris announced during a press conference that the Texas National Guard will immediately deploy an expected 250 peronnel with supporting aircraft, vehicles and equipment within 72 hours. (Photo by Drew Anthony Smith/Getty Images)
Trump: We are sealing up our southern border
01:30 - Source: CNN
CNN  — 

Hundreds of troops are on the way to the US-Mexico border after President Trump signed a memo warning of a security crisis there.

National Guardsmen are already meeting with US Customs and Border Protection officials to plan next steps.

They’ll work together, but in some ways troops will have to stay in the background; legal restrictions limit what they can do on US soil.

Defense Secretary James Mattis has authorized the deployment of up to 4,000 National Guard personnel through September 30, 2018.

What isn’t permitted? What can they do? And what questions are we still waiting for administration officials to answer?

Here’s a look:

What they can’t do

For Border Patrol agents, arresting undocumented immigrants and drug trafficking suspects is part of their job description. But National Guard troops will be limited in how much they can do to help.

There’s a law that dates back more than a century known as “posse comitatus,” which bars active-duty US troops from domestic law enforcement without authorization. Other laws and regulations have further clarified that troops can’t participate in activities such as making arrests and conducting searches, according to a Congressional Research Service analysis.

So how far can they go? Past presidents haven’t tested these boundaries and therefore courts haven’t weighed in. The Trump administration so far has avoided questions about how exactly it interprets these restrictions, saying lawyers are examining these issues.

Last week Defense Department officials said that without approval from Mattis, troops would not perform law enforcement activities or interact with migrants or others who are detained. Officials say the troops will only carry weapons in “circumstances that might require self-defense.”

In past border operations, officials have said National Guard troops can’t:

• Arrest people

• Seize drugs

• Conduct searches

What they can do

President George W. Bush sent 6,000 troops to the border. President Barack Obama sent 1,200. The deployments cost a total of more than $1.3 billion. In those operations, troops provided support to Border Patrol agents in a range of ways, including building and maintaining barriers and roads, conducting aerial surveillance and transporting agents.

This time around, the Defense Department says National Guard troops’ efforts will include:

• Aviation

• Engineering

• Surveillance

• Communications

• Vehicle maintenance

• Logistical support

Unanswered questions

Here’s what we don’t know yet – and hope to find out as troops begin arriving at the border:

• What exactly will they do?

• When will they be armed?

• Under what circumstances would Mattis approve troops performing law enforcement activities or interacting with migrants?

• How much will this operation cost?

• How many troops in total will deploy?

• Will officials end up extending the deployment beyond September?

CNN’s Tal Kopan and Barbara Starr contributed to this report.