In the incidents on October 14, a truck bomb exploded outside a busy hotel at the K5 intersection lined with government offices, restaurants and kiosks. A second blast struck Medina district two hours later.
The impact of the truck bomb was worsened by it exploding next to a fuel tanker that increased its intensity and left many bodies being burnt or mutilated beyond recognition.
By October 20, the government said the toll had reached 358.
It set up a committee, known as the Zobe Rescue Committee, to establish a more accurate death toll by talking to relatives of those who may have been at the site of the blasts.
"So far we have confirmed 512 people died in last month’s explosion ... (Some) 316 others were also injured in that blast," Abdullahi Mohamed Shirwac, the committee's chairman, told Reuters on Thursday.
There was no immediate comment from the government on the latest toll.
The bomb attacks were the deadliest since Islamist militant group al Shabaab began an insurgency in 2007. Al-Shabaab has not claimed responsibility, but the method and type of attack - a large truck bomb - is increasingly used by the al Qaeda-linked organisation.
Al-Shabaab stages regular attacks in the capital and other parts of the country. Although the group says it targets the government and security forces, it has detonated large bombs in crowded public areas before.
It has sometimes not claimed responsibility for bombings that provoked a big public backlash, such as the 2009 suicide bombing of a graduation ceremony for medical students.