Mexicans stand on the beach while looking through the U.S.-Mexico border fence into the United States on May 1, 2016 in Tijuana, Mexico. Mexicans on the Tijuana side can approach the border fence at any time. The U.S. Border Patrol, however, tightly controls the San Diego side and only allows visitors to speak to loved ones through the fence during restricted weekend hours at "Friendship Park, which straddles the border. #

A U.S. Customs and Border Protection helicopter pilot looks through night vision goggles while searching for undocumented immigrants under moonlight near the U.S.-Mexico border late on August 19, 2016, over Hidalgo, Texas. Air interdiction agents from Air and Marine Operations patrol areas near the Rio Grande, guiding U.S. Border Patrol agents on the ground as they pursue groups of immigrants and drug smugglers crossing illegally into Texas from Mexico. #

U.S. Border Patrol agents take undocumented immigrants into custody after capturing them after they crossed Rio Grande from Mexico into Texas on August 18, 2016 near Sullivan City, Texas. #

Central American immigrants wait to be transported after turning themselves in to U.S. Border Patrol agents on December 8, 2015 near Rio Grande City, Texas. They had just illegally crossed the U.S.-Mexico border into Texas to seek asylum. #

A pregnant Central American immigrant stands in line for a bus to an onward U.S. destination after being released by the U.S. Border Patrol on August 15, 2016 from McAllen, Texas. After crossing from Mexico into Texas, Central American immigrant families are processed by the U.S. Border Patrol center, given temporary legal documents and sent to their destination city, while their asylum petitions move through U.S. immigration courts. The families are assisted by the Sacred Heart Catholic Church Immigrant Respite Center in McAllen before their departure. #

A marker on the U.S.-Mexico border on September 23, 2016 in San Ysidro, California. Daily more than 10,000 people legally cross the border, mostly for work, at San Ysidro, making it the busiest port of entry on the 2,000 mile border between the United States and Mexico. #

A U.S. Customs and Border Protection officer checks identifications as people cross into the United States from Mexico on September 23, 2016 in San Ysidro, California. #

Maria Rodriguez Torres, 70, embraces a grandchild after seeing her other grandchildren for the first time through the U.S.-Mexico border fence on September 25, 2016 in Tijuana, Mexico. She had traveled with family members from Mexico City to see her grandchildren through the fence at "Friendship Park." #

Crosses reading "Hope" and "Not forgotten" in Spanish mark an immigrant's grave on September 27, 2016 in Holtville, California. Hundreds of immigrants, many who died while crossing the desert from Mexico into the United States, are buried in a pauper's cemetery. Many of the concrete grave markers simply read "John Doe." #

A digger removes sand drifts along the U.S.-Mexico border fence on September 28, 2016 in the Imperial Sand Dunes recreation center, California. Without daily removal of the sand, the dunes would cover the fence and undocumented immigrants and smugglers could simply walk over it. #

A U.S. Border Patrol Aerostat surveillance balloon flies near the U.S.-Mexico border on August 18, 2016 in La Joya, Texas. The Border Patrol has added additional Aerostats along the border, with high powered camera imagery assisting agents on the ground as they pursue illegal immigrants and drug smugglers. #

U.S. Border Patrol agents detain undocumented immigrants, one of them who needed medical attention, after capturing them in thick brush near the U.S.-Mexico border on December 10, 2015 at La Grulla, Texas. #

A U.S. Border Patrol agent leads undocumented immigrants through the brush after capturing them near the U.S.-Mexico border on December 7, 2015 near Rio Grande City, Texas. #

Ania, 9, from El Salvador walks with her father through the Texas countryside after crossing the Rio Grande from Mexico into the United States, to seek asylum in Roma, Texas, on April 14, 2016. #

Carlos, who lived undocumented in Los Angeles for 28 years before being deported to Mexico, touches fingers with his daughter, 9, through the meshed U.S.-Mexico border on September 25, 2016 in Tijuana, Mexico. U.S. Border Patrol agents allow people into San Diego's "Friendship Park" on weekends to meet through the fence with family and friends on the Mexican side. #

Family members embrace at the U.S.-Mexico Border fence under U.S. Border Patrol supervision during an "Opening the Door of Hope" event on April 30, 2016 in San Diego, California. Five families, with some members living in Mexico and others in the United States, were permitted to meet and embrace for three minutes each at a door in the fence, which the U.S. Border Patrol opened to celebrate Mexican Children's Day. It was only the third time the fence, which separates San Diego from Tijuana, had been opened for families to briefly reunite. The event was planned by the immigrant advocacy group Border Angels. #

The border fence stops at a hillside on the U.S.-Mexico border in Jacumba Hot Springs, California, photographed on September 26, 2016. The border stretches almost 2,000 miles, much of remote areas, and fencing often stops due to geographical features, such as hills and rivers. #