"In many instances, criminal organizations are saturating areas with large groups with the belief that they can smuggle narcotics or other contraband into the United States while Border Patrol agents are occupied", according to the news release.
CBP said it would release more information later Friday, but additional details had not been released by mid-afternoon.
Officers of the Mexican equivalent of U.S. Customs and Border Protection reportedly surrounded the truck and discovered the driver dead. There were almost 150 children, including 32 who were traveling alone.
Border Protection and Nogales police officials both alerted that the port of entry's outbound lanes were shut down due to an emergency, but did not elaborate.
The vote comes after US military troops installed additional layers of wire along the fence over the weekend, and the day after President Donald Trump highlighted border security in his State of the Union address. In that area of the border, there is only a barrier to prevent vehicles from coming across. It was the second straight monthly decline, though arrests typically fall from December to January.
The 290 migrants included people from Guatemala, Nicaragua, El Salvador and Honduras.
The January numbers are unlikely to swing public opinion either way in President Trump's showdown with Democratic leaders in Congress over funding to replace and extend border walls with Mexico. Customs and Border Protection says medical attention was needed about 12,000 times for border crossers in the 12-month period ending September 30.