The FBI added the names of nine Chinese people and one other man to the list of those being sought for questioning about a possible terror plot targeting Boston.
FBI spokesman Joe Parris said on Thursday the names "were developed as a result of the ongoing investigation" but did not signal that credible evidence has emerged indicating such a plot actually exists.
"Information is still uncorroborated and from a source of unknown reliability and motive," Parris said.
The names are part of the same anonymous tip that led authorities on Wednesday to announce that they are seeking to question four other Chinese and two Iraqis. The new names bring to 16 the people being sought for questioning.
Another federal law enforcement official in Washington, speaking on condition of anonymity because of the ongoing investigation, said the tip was received by the California Highway Patrol. The tipster claimed the four Chinese -- two men and two women -- entered the US from Mexico and were awaiting a shipment of "nuclear oxide" to follow them to Boston.
The official said the nuclear oxide could be a reference to material used to make a "dirty bomb" that would spew radiation over a wide area.
US Attorney Michael Sullivan of Boston said earlier on Thursday that authorities had learned more background about the original four Chinese, but "it makes us no more alarmed this morning, this afternoon, than we were yesterday."
"They're not wanted at this point in time for any crimes because there's no evidence at this point in time that they've committed any crimes," Sullivan said. "We're not certain exactly where they are. We can't even say for certain that they're in the country."
White House Chief of Staff Andrew Card said he discussed the manhunt with US President George W. Bush during an Oval Office meeting a few hours before Bush's inauguration for a second term. Card did not provide details on what was said.
The four Chinese previously named by the FBI were identified as Lin Zengrong, Wen Quin Zheng, Chen Xiujin and Lin Guozhi. Authorities said none of the names had appeared on previous watch lists of terror suspects. The bureau also released pictures of those four but not of the others being sought.
A woman was among the nine new Chinese names: Yu Xian Weng, a woman 41 years old. The others were all men: Lin Quinquan or Quiquan, 21; Liang Liqiang, 28; Min Xiu Xie, 27; Xiang or Xing Wei Liu, 22; Mei Xia Dong, 21; Chen Xiuming; Cheng Yin Liu; and Zao Yun Wang.