(CNN) President Donald Trump has approved a memorandum that grants new authority to US troops on the Southwest border to protect Customs and Border Protection personnel from migrants if they engage in violence, according to the Pentagon.
Department of Defense spokesperson Lt. Col. Michelle Baldanza confirmed to CNN that the Pentagon had received the memorandum.
CNN first reported on Monday that this was expected.
The White House memo authorizes US troops to conduct activities such as "crowd control, temporary detention and cursory search" according to a copy of the memorandum, which has been obtained by CNN.
It allows troops to use "a show or use of force (including lethal force, where necessary)" in order to perform these protective activities.
While some have questioned whether active duty troops detaining and searching people on US soil constitutes law enforcement, something prohibited under the Posse Comitatus Act, the memo, which is signed by White House Chief of Staff John Kelly, says that US troops "shall not, without further direction from the President, conduct traditional civilian law enforcement activities, such as arrest, search, and seizure."
"On detention we do not have arrest authority, detention -- I would put it in terms of minutes, in other words if someone's beating on a border patrolman and if we were in a position to have to do something about it we could stop them from beating on them and take them over and deliver them to a border patrolman who would then arrest them for it," Secretary of Defense James Mattis told reporters at the Pentagon Wednesday when asked about the new authorities.
"There's no violation of Posse Comitatus, there's no violation here at all we're not going to arrest or anything else," Mattis added.
The memo is addressed to Mattis, the attorney general and the secretary of homeland security.
The document cites "credible evidence and intelligence" indicating that the migrant caravans originating from Central America "may prompt incidents of violence and disorder that could threaten US Customs and Border personnel" as the reason why US troops are needed to help protect them.
The decision comes as a large group of migrants is arriving at the US-Mexico border, having been the focus of Trump's closing message during the midterm election. In the days and weeks leading up to the election, Trump ordered military troops to the border to assist the Department of Homeland Security with preparing for the migrants' arrival.
"I'm here to tell you today that the reality is that there are currently over 6,200 individuals camped out south of the US border in Tijuana today, as I stand here," Homeland Security Secretary Kirstjen Nielsen said at a speech along the border earlier this week. "In Mexicali there are more than 3,000 caravan members. These two groups combined with other caravans making their way though the Northern Triangle and Mexico will result in approximately eight to ten thousand migrants amassing along our southern border. The crisis is real, and it is just on the other side of this wall."
Until this new authority was granted, troops were not allowed to intervene if CBP personnel came under attack unless they needed to act in their own self-defense.
Tuesday's authorization comes after Department of Homeland Security officials said Monday that they were getitng information from "multiple sources including individuals in the Mexican government" of potential waves or groups of individuals who were discussing an incursion into legal ports of entry in California by attempting to pass through vehicle lanes.
The Pentagon has been working for the last several days on options for how troops can protect CBP. There are 5,800 to 5,900 troops assigned to the border mission.
Any potential use of force by US troops to protect CBP personnel must be "proportional," a US official previously told CNN.
Mattis said Wednesday that "there has been no call for any lethal force from DHS," saying that any troops backing up Customs and Border Patrol would not be carrying firearms but could be equipped with shields and batons.
Two defense officials had also emphasized that National Guard forces activated by governors, as well as state and local civilian law enforcement authorities in a given area, should be relied on as much as possible.
All three officials were adamant that the change is not about troops firing weapons at migrants crossing the border. Instead the new rules will be aimed at providing the basic authorities to allow for protective measures.
The Pentagon said Tuesday the mission will cost taxpayers approximately $72 million. That price tag covers the cost of deploying the approximately 5,900 active duty troops until December 15 and will increase the longer the troops are deployed.
Mattis said the cost was likely to increase, saying he was "confident that number will go up."
Trump has said he will deploy as many as 15,000 troops if needed to push back against the group of migrants who are planning on asking for asylum. On Tuesday, Trump told reporters that the troops "are proud to be on the border. They are proud to be defending our nation. And we are not letting people in."
Earlier this year, CNN reported that the Pentagon had rejected a similar request from the administration to send troops to protect CBP personnel at the border on the grounds that the soldiers would lack the proper authority to do so.
A separate defense official tells CNN that planning is underway to move some military police from Texas to California, where the majority of migrants trekking through Mexico toward the US are expected to go.
Nielsen addressed the possible movement of troops earlier Tuesday, saying the completion of some military missions had resulted in a "movement of forces."
"There's some missions that parts of the military have completed, so what you are seeing is a movement of forces," Nielsen told reporters. "But as I said earlier, I have spoken with Secretary [of Defense Jim] Mattis directly and I have no doubt they will continue to partner with us in this mission until it is resolved."
This story has been updated with additional developments.