"Supposedly there were many witnesses so the government does not have to rely on a confession from him. So it remains to be seen what shape any plea would take."
Protest outside court
Ackerman said Abdulmutallab was indicted for attempted murder of the 290 people on board the plane.
"But this charge does not carry the longest sentence. Rather, the most serious of the six charges is trying to use a weapon of mass destruction", he said.
Ackerman said that Abdulmutallab may also be detained indefinitely for illegal alien entry to the US.
He added that about 100 residents of Detroit - which has one of the largest Arab communities outside the Middle East - demonstrated outside the court in solidarity with the US government and against al-Qaeda who they said were trying to smear all Muslims.
Allegedly linked to al-Qaeda, Abdulmutallab's failed attack on a flight from Amsterdam to Detroit has triggered worldwide security alarm.
In particular, it has led the United States to adopt stringent new screening and security measures at airports. Dozens of names have also been added to no-fly lists.
Barack Obama, the US president, on Thursday ordered a sweeping overhaul of flawed intelligence services but said "ultimately the buck stops with me".
Obama said intelligence agencies had the necessary information to prevent the bomb attempt but failed to connect and understand the disparate pieces of data.
The measures outlined at the White House included wider and quicker distribution of intelligence reports and stronger analysis of them, as well as tightened passenger screening and expanded watch lists.
But he warned that "there is, of course, no foolproof solution".
The White House also released a report on Thursday of how the alleged bomber in the December 25 failed attack managed to elude the authorities and board the Detroit-bound plane with explosives.
The six-page summary of the report given to Obama stated that US intelligence officials had received unspecified "discreet pieces of intelligence" as early as October to identify Abdulmutallab as an al-Qaeda operative and keep him off the plane.
But officials did not increase their focus on the threat and did not pull together fragments of data needed to foil the scheme, said the summary.
Obama said the "incident was not the fault of a single individual or organisation, but rather a systemic failure across organisations and agencies".
"I am less interested in passing out blame than I am in learning from and correcting these mistakes to make us safer. For ultimately the buck stops with me," he added.
"As president I have a solemn responsibility to protect our nation and our people, and when the system fails, it is my responsibility."