Arrested on Saturday, a day after two bombs mailed to the US and carried on cargo planes sparked global fears of a major terror threat, Hanan al-Samawi — a 22-year-old student at a university in Yemen — and her 45-year-old mother were released on Sunday.
Hanan was reportedly freed on condition she make herself available for further questioning in the foiled terror plot that authorities in the US said bore the "hallmark of al Qaeda".
The bomb is being called the handwork of Ibrahim Hassan al-Asiri, al Qaeda's top bombmaker in Yemen.
He was also behind the Christmas bombing attempt in 2009, in which a Nigerian tried to blow up a plane.
The explosives were sewn into his underwear.
Asiri is also believed to be behind the attempt to kill the Saudi Arabian intelligence chief.
Asiri had hidden the explosives into the body of a suicide bomber, his own brother. Investigators are also looking into the possible role of local language schools in Yemen.
The bombs, which were found during scanning of cargo shipment in Dubai and Britain on Friday, were made of PETN, packed into Hewlett-Packard printer cartridges with remarkable sophistication, investigators have said.
One of the two mail bombs had reportedly travelled on two separate passenger flights before being found, a spokesman for Qatar Airways said on Sunday.
The Qatar Airways spokesman said the device discovered in Dubai was put on a passenger flight between Sanaa and Doha, in Qatar, before being shipped in a separate plane to Dubai, where it was discovered.
The bombs were headed for Jewish community centres — synagogues — in Chicago, President Barack Obama's hometown.
American officials have said they suspect the bombing attempt had the backing of the top leadership of al Qaeda in the Arabian Peninsula, including the radical American-born cleric Anwar al-Awlaki.
The students at Sanaa University had protested the arrest of al-Samawi, saying she was innocent. Officials said she had been traced through a telephone number she had left with a cargo company.
(With agency inputs)