A three-year-old migrant boy was found wandering alone in a Texas cornfield near the US-Mexico border.
The boy, in tears, was discovered in the Rio Grande Valley near Brownsville by US Border Patrol agents on Tuesday morning.
Officials said his name and a telephone number were written on his shoes.
The US Customs and Border Protection (CBP) tweeted: “We believe the boy was with a larger group that ran when they encountered agents.”
The boy has been taken to a nearby border station while officials try to trace his family.
It came as Donald Trump renewed his threat on Wednesday to send more troops to the US-Mexico border after an incident in which Mexican soldiers confronted US personnel.
Mexico blamed the incident on confusion, and said it was not looking for confrontation with the US.
In tweets on Wednesday morning, Mr Trump said: “Mexico’s Soldiers recently pulled guns on our National Guard Soldiers.”
He said it was probably as a diversionary tactic for drug smugglers on the border.
The president added: “Better not happen again! We are now sending ARMED SOLDIERS to the Border. Mexico is not doing nearly enough in apprehending & returning!”
US Border Patrol agents have been struggling to cope with the rising number of families trying to reach the US from Central America.
Between January and the end of March this year, 207,475 people have been detained on the southwest US border, according to US Border Patrol figures.
Mr Trump made migration a key part of his presidential campaign as he said criminals and drug dealers from Mexico were swamping the US.
The president had promised to build a wall to stop the flow of migrants – but last year, Congress refused to give him the $5.7bn (£4.5bn) needed to construct it, leading to a government shutdown.
In February, he declared a national emergency so he could get round Congress and divert funds from the military to the wall.
In March, army engineers received permission to start planning and building 57 miles of 18ft-high fencing along the border.