Security officials said one detainee had died from an asthma attack, while Sahwa Net, an opposition website, said he died from wounds after being tortured.
The defence ministry said security forces had received information from Ghodel Mohammed Saleh Naji, described as a previously detained al-Qaeda operative, that those responsible for the attack in Aden would be holding a Friday meeting.
All participants in the meeting were held, the statement said. Authorities said earlier they had arrested the head of the group behind the assault.
Yemen is under international pressure to quell domestic conflicts to focus on an expanding al-Qaeda presence in the country.
The country's Western allies and neighbouring Saudi Arabia have long feared a resurgent al-Qaeda wing could take advantage of rising insecurity and weak central control to use Yemen as a base for destabilising attacks in the region and beyond.
Al-Qaeda and the Yemeni government have clashed for many years, but the group's major operations have typically focused on Western targets, such as a failed attempt to assassinate the British envoy to Sanaa, the capital of Yemen, in April.
An al-Qaeda attack on the US embassy in Sanaa in 2008 killed 16 people, including six of the assailants.