The Federal Bureau of Investigation (FBI) in the United States has named a Pakistani woman among seven dangerous al-Qaeda suspects plotting an attack.
Miss Siddiqui as seen on the FBI website
The FBI says that that Aafia Siddiqui could be planning strikes in the US or against US targets overseas.
A former student from the city of Boston, Miss Siddiqui is now reported to be underground in Pakistan.
But Pakistani officials say they have no idea where she is, and efforts to trace her have not been successful.
'High profile strike'
FBI Director Robert Mueller warned on Wednesday that the forthcoming dedication of a World War Two memorial in Washington, the Group of Eight summit in Georgia and the conventions of the two main American political parties could all be targeted.
Miss Siddiqui obtained a biology degree in the US in 1995 and is reported later to have become a doctor in neurological science.
Adna El Shukrijumah is reported to have links with Miss Siddiqui
The American media has reported that she is linked with Adna El Shukrijumah, another one of the key seven suspects named by the FBI.
Pakistani officials have meanwhile said they were making efforts to trace Miss Siddiqui.
"We have tried to locate her, but she has apparently gone underground. We do not know why," a senior Pakistani security official told the Associated Press news agency.
"It is not in our knowledge if she in Pakistan or somewhere else," he said.
Miss Siddiqui's mother said in 2003 that she had last seen her daughter in April of that year when she took a minicab from Karachi to Islamabad.