The bodies of 59 suspected illegal migrants have been found in the Arizona desert on the border with Mexico in July, the highest number in five years, officials say.
High temperatures last month are likely to have contributed to the death rate.
But some analysts say toughened border controls have forced more would-be migrants into riskier desert crossings.
The Pima County medical examiner's office is using a refrigerated trailer for the dead as its morgue gets full.
Arizona media reported that July's monthly total was the highest since July 2005, when the Pima County medical examiner recorded 69 deaths in the desert.
Smugglers are leading illegal migrants through remote desert corridors far from border patrol agents and check points, adding to the risk, border patrol spokesman Omar Candelaria told the Arizona Daily Star newspaper.
Thirty-two of the 59 people died from hyperthermia or heat exposure, Pima County medical examiner records show.
Most of the dead remain unidentified. Not all of the remains are thought to date from July.
Pima County sits along 123 miles (198km) of the US border with Mexico.
Hundreds of migrants die trying to cross the US border each year after being led into the desert by trafficking gangs.