Border Patrol officials from San Antonio and Laredo responding late Friday afternoon to reports of a human smuggling operation.
A big rig was stopped just north of Pearsall at a truck stop on I-35. Pearsall is Southwest of San Antionio.
Immigration and Customs Enforcement officials confirmed that immigrants -- maybe dozens -- were found in the back of a semi truck.
An ICE spokesperson said special agents were on the scene along with Border Patrol agents from Laredo.
An official from Frio Regional Hospital confirmed they were treating some of the immigrants and that others were taken to hospitals in that region.
There was no word on what tipped off authorities to the illegal transport or what happened to the driver of the truck.
An attendant at the truck stop told a KENS 5 reporter that she saw local, state and federal agents on the scene at the time of the bust.
None of the immigrants were believed to have died, according to ICE; however, there was no word on their conditions Friday evening.
A Border Patrol official confirmed the incident to CBS News late Friday:
"On September 18, at around 4p.m., Cotulla Border Patrol Station received a call for assistance from Frio County Sheriffs Office regarding a concerned citizen call they received of several people locked inside a tractor trailer near a gas station on HWY 57 and 35. Upon arrival to the scene, Border Patrol agents assigned to the Cotulla Station assisted Frio County Sheriff's Deputies in taking custody of the people. A total of 37 undocumented aliens were located inside the trailer. EMS was requested and transported the people to several area hospitals for medical treatment. The case is currently under investigation by Homeland Security Investigation. "