Fifty-one migrants, including 21 children, were found hidden in secret compartments of a truck in Mexico.
Officials used a large X-ray scanner at a checkpoint which uncovered the people who were mostly from central America.
Footage of the X-ray shows a ghost-like truck filled with the brightened silhouettes of the migrants, some sitting, others lying down along two levels in the crowded compartments.
The migrants from Guatemala, Honduras, El Salvador, Nicaragua and Ecuador, 21 of whom were minors, are now in the custody of the country's National Migration Institute (INM), government officials said in a statement late on Sunday.
The migrants will be returned to their home countries, the statement said.
The apprehension comes weeks after the appointment of hard-charging former prisons chief Francisco Garduno as the new head of the INM.
Under Garduno, the immigration authority has made a number of high-profile captures of migrants smuggled in vehicles.
The INM's former chief official, an academic recognized for his work on immigration policy, left his position in mid-June amidst tensions over a June 7 deal with the United States under which Mexico averted tariffs threatened by President Donald Trump.
In return, Mexico promised to make palpable progress in reducing the number of mainly central American migrants trying to cross the border into the US illegally.
In another video of the X-ray-assisted detention released by Mexican authorities, an official approached the cargo vehicle at night to open the door to one of the crates.
Sobs emerged from the otherwise-quiet container while he identified himself as a police officer and asked if migrants were in good health.
The truck, the driver, and 225,000 Mexican pesos ($12,000) in cash found in the vehicle are now in police custody, and will be subject to investigation, INM said.
Mexico's Foreign Minister Marcelo Ebrard said the attorney general has about 11 open investigations into migrant smugglers.
"It is our strategic objective to end the impunity for human traffickers," Ebrard said, at a news conference on Monday.
