The lawyer, Brandon Mayfield, was the first person in the United States to be drawn into the investigation of the Madrid bombings, which killed 191 people and which Spanish officials have linked to possible associates of Al Qaeda.
The F.B.I. executed a search warrant on Mr. Mayfield’s home in the Portland area and a second one at his law office, said the government official, who spoke on condition of anonymity. Officials would not say what agents were searching for or what led them to Mr. Mayfield.
“All I can say is we served two search warrants,” said Beth Anne Steele, a spokeswoman for the F.B.I. in Portland. She declined further comment.
Mr. Mayfield’s mother and his former step-mother, who both live in Kansas where Mr. Mayfield was raised, were questioned Thursday by F.B.I. agents, the former step-mother, Ruth Alexander, said. Ms. Alexander said she had helped raise him from the age of 12.
Both of the women were stunned, said Ms. Alexander, who said she had learned about Mr. Mayfield’s arrest when F.B.I. agents arrived at the auto repair shop where she works as an officer manager in Halstead, Kan.
“This somehow has to be some kind of a mistake,” Ms. Alexander said in a telephone interview. “I think they’re grasping at straws to try and nail people for some of these crimes. And no one, no one, can imagine that he would have anything intentionally to do with anything.”
Outside the Mayfields’ home in the Portland suburb of Aloha, his wife, Mona Mayfield, gave a brief statement to reporters. “I know he is innocent,” she told KGW, a Northwest cable news station. “Everyone knows he’s innocent.”
Few details emerged Thursday night about Mr. Mayfield’s case.
A government official said: “We don’t know how strong the case is, and the information is very limited. It’s a very active investigation at this point.”
The use of a material witness warrant suggested that authorities were still building their case against Mr. Mayfield, 37. The Justice Department has used the warrants with increasing frequency since the Sept. 11 terrorist attacks to indefinitely incarcerate dozens of people who the authorities say may have had information relevant to an investigation. Some civil rights advocates have attacked the practice, saying it has been abused.
Mr. Mayfield, who joined the Army after high school, partly to pay for his college tuition and, later, law school, Ms. Alexander said, served at Fort Lewism, Wash., where he met his wife, an Egyptian. He passed the bar in Portland in 2000. He ran his own business, with his wife working as the secretary, Ms. Alexander said, and was “struggling” to support three children, between the ages of 10 and 15, on the income from a start-up law firm.