"The new prison wing cost some $60 million to build ... and is meant to be part of a new era of openness and transparency," Bays said.
"But we were not shown the detainees. Human-rights lawyers say that, while the environment for the prisoners may be changing, their legal situation is not ... not having been charged. Nor has any civilian lawyer ever been allowed inside."
Bays said the extended prison could hold up to 1,000 detainees, but was at present holding around 700 inmates, including 30 foreign prisoners.
Omar Dighayes, a former detainee at Bagram and Guantanamo Bay, said the Bagram prison resembled a concentration camp.
"People were beaten, dragged, tortured in it. There were high places where guards stood with guns. It was a hard, difficult place," he told Al Jazeera.
But he said he doubts the newly refurbished Bagram prison will improve conditions for its detainees, one of which includes his brother-in-law, whom Dighayes says was recently "badly beaten" inside Bagram.
"I don't think it's the facilities which make the difference, it's the treatment of people inside.
"Everybody who worked in Bagram - from the American side - will tell you that the things I'm describing did happen. People from the military intelligence [and] people from the FBI have spoken about the barbaric treatment at this facility."
But General Mark Martins, who runs detention operations at the airbase, said the US military was improving its treatment of detainees and had learnt many lessons since occupying the country in 2001.
"Detention, if not done properly, can actually harm the effort. We are a learning organisation ... we believe transparency is certainly going to help the effort, and increase the credibility of the whole process," Martins said.
'Guantanamo's evil twin'
However, Clara Gutteridge, an investigator of secret prisons and renditions from the human rights organisation, Reprieve, said Bagram is seen as "Guantanamo's lesser-known evil twin".
"All this talk about transparency, and the US government still won't release a simple list of names of prisoners who are in Bagram," she told Al Jazeera.
"None of them have had access to a lawyer ... and that just seems very unfair.
"We at Reprieve see this as the next big fight after Guantanamo Bay.
"But one thing that the US government is saying is that Afghan prisoners in Afghanistan have less rights than any other prisoner which just seems absurd."
Bagram Air Field is the largest US military hub in Afghanistan and is home to about 24,000 military personnel and civilian contractors.
Tens of millions of dollars continue to be spent on expanding and upgrading facilities - turning Bagram into a town spread over about 5,000 acres.
The air field part of the complex is already handling 400 tonnes of cargo and 1,000 passengers daily, according to Air Force spokesman Captain David Faggard.
It is continuing to grow to keep up with the requirements of an escalating war and troop increases.
"Detention, if not done properly, can actually harm the effort"
General Mark Martins,
commander of detention operations
Among new options being considered in Washington is regional commander General Stanley McChrystal's request to bring an additional 40,000 troops to Afghanistan.
But even with current troop levels - 65,000 US troops and about 40,000 from allied countries - Bagram already is bursting at the seams, our correspondent reported.
Plans are under way to build a new, $22m passenger terminal and a cargo yard costing $9m. To increase cargo capacity, a parking ramp supporting the world's largest aircraft is to be completed in early 2010.
Bagram was previously a major Soviet base during Moscow's 1979-89 occupation of Afghanistan, providing air support to Soviet and Afghan forces fighting the mujahidin.
Bagram lies in Parwan, a relatively quiet province. The Taliban is not believed to have a significant presence in the province.
But the base is susceptible to rocket and mortar attacks. In 2009, the Taliban launched more than a dozen attacks on the base, killing four and wounding at least 12, according to Colonel Mike Brady, a military spokesman.