The security checks at the airport -- and elsewhere in the Rio Grande Valley -- are familiar to people living along the Texas-Mexico border.
Along highways out of the area, drivers are stopped at Border Patrol checkpoints about an hour's drive north of the border. And it's not uncommon for children who entered the country illegally with their parents to grow up in the Rio Grande Valley to stay home when classmates go on field trips along those roadways to San Antonio.
In recent years, some U.S. citizens who object to being asked about their citizenship at the interior checkpoints have taken to refusing to answer agents' questions or produce identification while recording video that is later uploaded to the Internet.
Vargas, a native of the Philippines, was unaware he would have to pass through an immigration check prior to arriving in the city, said Ryan Eller, campaign director for Define American, the advocacy group founded by Vargas.
"We had been to border towns before like San Diego and other places, but we didn't recognize until here the situation," Eller said while standing across the street from the Border Patrol station where Vargas was being held. "We tried to prepare for basically every scenario that we could."
Vargas' last tweet Tuesday morning was a photograph of his Philippines passport and a palm-size copy of the U.S. Constitution.
Eller confirmed that the only identification Vargas carried was that passport. He said Vargas was en route to Los Angeles and that he had consulted with attorneys before going to the airport. Eller said a "travel partner" was at the airport with Vargas, but that they were immediately separated in security.
Vargas' attorney didn't immediately return messages for comment.
Vargas had flown to McAllen last Thursday to take part in the vigil. In an essay he wrote for Politico on Friday, Vargas said he has traveled in the U.S. for years without a problem but didn't realize that immigration checks are done on those driving or flying out of the Rio Grande Valley in South Texas. Vargas noted that he doesn't have any government-issued U.S. identification.
Eller said they were asking President Barack Obama and Homeland Security Secretary Jeh Johnson to use their authority to immediately release Vargas. White House press secretary Josh Earnest declined to comment Tuesday on Vargas' detention.
Vargas went public about his immigration status in a 2011 piece for the New York Times Magazine. He was part of the Washington Post team that won the Pulitzer Prize in 2008 for coverage of the Virginia Tech massacre.
He also directed a documentary called "Documented," and founded the activist group "Define American."
Vargas attended school in Mountain View and graduated from San Francisco State University.
Here are Vargas' two last Tweets today ...
Here is Vargas talking to KQED Newsroom host Thuy Vu in May...