Arizona and Texas have announced that they will send 400 National Guard members to the US-Mexico border by next week in response to President Donald Trump's call for troops to fight drug trafficking and illegal immigration.
Arizona governor Doug Ducey said about 150 Guard members would deploy next week. And the Texas National Guard said it was already sending Guardsmen to the border, with plans to place 250 troops there in the next 72 hours. Two helicopters lifted off Friday night from Austin, the state capital, to head south.
The total remains well short of the 2,000 to 4,000 National Guard members that Mr Trump told reporters he wants to send. New Mexico governor Susana Martinez's office said that it had not yet deployed any Guard members. The office of California governor Jerry Brown did not respond to questions about whether it would deploy troops.
Mr Trump's proclamation on Wednesday directing the use of National Guard troops refers to Title 32, a federal law under which Guard members receive federal pay and benefits, but remain under the command and control of their state's governor. This leaves open the possibility that California's Brown could turn him down.
Deployments to the border under former presidents George W Bush and Barack Obama both occurred under Title 32. Mr Bush sent around 6,000 troops in 2006, and Mr Obama sent 1,200 Guard members in 2010.
Mr Trump's proclamation blamed "the lawlessness that continues at our southern border". Mr Trump has suggested he wants to use the military on the border until progress is made on his proposed border wall, which has mostly stalled in Congress.
After plunging at the start of Mr Trump's presidency, the numbers of migrants apprehended at the southwest border have started to rise in line with historical trends. The Border Patrol said it caught around 50,000 people in March, more than three times the number in March 2017. That has erased a decline for which Mr Trump repeatedly took credit. Border apprehensions still remain well below the numbers when Bush and Obama deployed the Guard to the border.
News reports of a caravan of Central American migrants passing through southern Mexico also sparked angry tweets from the president. The caravan of largely Central American migrants never intended to reach the US border, according to organiser Irineo Mujica. But Mr Trump has repeatedly cited it as an example of what he called America's weak immigration laws.
Department of Homeland Security officials have said Guard members could support Border Patrol agents and other law enforcement agencies. DHS Secretary Kirstjen Nielsen said this week that guard members could "help look at the technology, the surveillance", and that the department might ask for fleet mechanics. Federal law restricts the military from carrying out law enforcement duties.
The Arizona National Guard said in a statement that it would "provide air, reconnaissance, operational and logistics support and construct border infrastructure". Leaders in both Arizona and Texas said they were working with federal planners to define the Guard members' mission.
From 2006 to 2008, the Guard fixed vehicles, maintained roads, repaired fences and performed ground surveillance. Its second mission in 2010 and 2011 involved more aerial surveillance and intelligence work.
Former Texas governor Rick Perry, now Mr Trump's energy secretary, also sent about 1,000 Guard members to the border in 2014 in response to a surge in the number of unaccompanied immigrant children crossing the Rio Grande, the river that separates the US and Mexico in the state.
About 100 Guardsmen remain deployed as part of that existing state mission.