The scanning equipment delivering the images to his computer is part of Mexico's new bid to stop undocumented migrants and human traffickers.
It is a crackdown that is about to get stronger, under the deal the Mexican government struck Friday with the United States to avoid President Donald Trump's threatened tariffs.
At another checkpoint nearby, officers inspect minibuses and taxis heading north from the Suchiate River, the frontier between Mexico and Guatemala.
It does not take them long to find an undocumented family traveling in a minibus. They make them get off -- a father, mother and three children, including a baby -- and put them in a van with bars on the windows.
It is likely the first step toward deportation.
"We're here 24/7," one border officer tells AFP, speaking on condition of anonymity.
And the Central American migrants fleeing in droves from desolate poverty and brutal street-gang violence have begun to feel the impact.
Migrant detentions have tripled in Mexico since January, to 23,679 in May. Deportations are also up, to 16,507 last month.
As part of its deal to avert Trump's threat of five-percent tariffs on all Mexican goods -- which would have risen incrementally to 25 percent by October -- President Andres Manuel Lopez Obrador's government has agreed to deploy 6,000 officers from Mexico's new National Guard to the southern border starting Monday.
That will make it that much harder for Central Americans to slip through what has traditionally been a highly porous border.
Though some migrants and activists remain defiant.
The new deployment "might make the big caravans disappear, but not migration in general. It will continue one way or another," said Ernesto Castanedo, an activist at the Buen Pastor migrant shelter in the southern city of Tapachula.
The shelter houses 600 migrants, mostly Hondurans taking a break on their northward trek.
The migrants watch TV, play football and wash their clothes as Unicef workers give their children impromptu classes.
"It doesn't matter what Donald Trump does, he can never stop us," said another migrant, 57-year-old Josue Arenal, from Honduras.
"Migrants always sneak through. He can close the border, build a thousand walls, migrants are always going to find a way."
Contraband of all sorts
AFP reporters saw undocumented migrants crossing the border at dawn on rafts, a day after the US-Mexican deal was announced.
The customs officers on the bridge appeared indifferent to the thriving black-market operations ferrying migrants and stolen gasoline across the river.
"They should go after organized crime," not migrants, said Abraham, 49, a one-time migrant from El Salvador who now works in a migrant shelter.
He experienced first-hand the dangers migrants face when he made his own attempt to reach the United States by hopping the freight train known as "The Beast."
Members of a Mexican gang -- one of many known for preying on migrants -- robbed him and threw him off the train.
Severely injured, he spent five years recovering in hospital, before returning to El Salvador.
When he made it back to his village, he found his family had already given him up for dead. At the local cemetery, they had erected a tombstone with his name on it over an empty grave.