Jose Ernesto Beltran Quinones was taken into custody Monday in Mexicali by Mexican state judicial police, Dan Dzwilewski, special agent in charge of the FBI's San Diego division, told the San Diego Union-Tribune.
Quinones, one of the, is being questioned on behalf of the FBI, according to special agent Kiffa Shirley.
"The first area of concern for the FBI is to resolve any pending national security threat issues, and that issue being the statement that was made that nuclear material was being brought into the United States," Dzwilewski said. "We're working with Mexican authorities trying to resolve that question."
Shirley said late Monday he did not know Quinones' nationality, age or occupation, or where he is being questioned.
Officials have stressed since news of the tip first broke that they doubted the credibility of the terror claims. A leading theory is that a smuggler tipped authorities to a false terror plot to exact revenge on a group of Chinese immigrants, perhaps because members failed to pay.
The tipster claimed that members of a group of 16 people - 13 Chinese nationals, two Iraqis and one other person whose nationality was not released - had talked about a shipment of material supposedly called "nuclear oxide" that would follow them from Mexico to Boston. The implication was that the group was plotting to detonate a radioactive "dirty bomb" that spews hazardous material and can sicken or kill people.
No evidence has been found for such a plot.
Last Wednesday's tip on a possible threat sent a wave of fear through Boston, as authorities ramped up security just in case, distributing photos of wanted suspects, searching public transportation and putting radiation monitors in key locations.
At Logan Airport, where two of the planes were hijacked for the Sept. 11 terrorist attacks, the tip has been taken seriously, according to Dennis Treece, director of corporate security. The most visible sign is more patrols.
Pictures of four Chinese suspects released by the FBI were taped inside booths where subway tokens are sold by transit employees, and operators of underground parking garages have been searching vehicles.
Massachusetts Gov. Mitt Romney - who was in Washington for President Bush's inauguration - wound up flying home Wednesday, before the ceremony, to be ready to manage whatever might happen.
Dzwilewski said the United States would like to extradite Quinones, who Shirley said is being interviewed as part of a joint investigation by the FBI and Mexican authorities.
"We're so pleased with the extraordinary cooperation of Mexican authorities," Shirley said.
Over the weekend, the FBI said another person who had been wanted for questioning in relation to the alleged plot has been in federal custody for more than two months and has no terrorist connections. She is identified as Mei Xia Dong, 21, of China.
Bomb expert Charles Ferguson says if the threat is real, several radioactive compounds take form as oxides and could be used in a so-called "dirty bomb." Plutonium and americium oxides, in the right amounts, would be dangerous to human health, while uranium oxide would be less so, he said.
"They vary in potency," said Ferguson, science and technology fellow at the Council on Foreign Relations in Washington. "If it was plutonium, we could have a problem on our hands."