Police found the trailer Sunday afternoon at a rest stop in Edinburg, Texas, after a relative in Mexico called authorities.
Upon arrival, first responders began knocking on trailers parked around the rest stop until they located the immigrants, who came from Mexico, Guatemala, Honduras and Romania.
A man and a woman were detained in connection with the incident.
The condition of the people who were found inside the trailer is not known.
Sunday's discovery comes less than a month after more than 30 people were found inside a trailer in San Antonio.
Ten people died after being smuggled into the country in that case from heat exhaustion.
The driver in that case, James Matthew Bradley, 60, is now facing federal smuggling charges.
Driver charged in human smuggling case says he had 'no idea' what was in truck
Survivor in human smuggling case says people cried, asked for water
Survivor says trailer was full and had no water
What the driver claimed happened in the immigrant smuggling attempt gone wrong
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