Government inspectors first became aware of the missing shipment about a week after it crossed the border, in early June, officials said Friday.
Sensors to detect material at the border showed positive readings when checked days after the truck apparently passed, the Detroit Free Press reported Saturday.
White House officials said they were taking the missing shipment seriously, but pointed out the material could have been for a legitimate purpose, such as construction or medical supplies. They also said sensors could have shown a false positive reading.
"There is no intelligence information that indicates this is related to terrorism," said White House Office for Homeland Security spokesman Gordon Johndroe. "We have no credible intelligence that indicates al Qaeda or any other terrorist organization has smuggled radioactive material into the country."
Department of Energy nuclear emergency support teams have been searching Michigan and the northern Midwest, officials said.