"The FBI was present near Windridge Drive and Estate Drive in Laredo and carried out law enforcement activity," the FBI said in a statement. "The FBI can not comment further on an ongoing investigation."
A spokesman for Cuellar told CNN that "Congressman Cuellar will cooperate fully in any investigation. He is committed to ensuring that justice and the law are upheld."
Cuellar, a moderate Democrat, has served in Texas' 28th Congressional District since 2005.
The news was first reported by the newspaper The Monitor.
Reporter Valerie Gonzalez with The Monitor reported in a tweet Wednesday night that "around 7:30 p.m., a group of agents loaded large bags, plastic containers and a computer into a federal vehicle. Others appeared to be logging the items."
This story has been updated with further details on Wednesday.