The panel said they were "dismayed" that documents they had requested
about the country's air defences on the day of the attack had not been
provided.
They subpoenaed the Federal Aviation Administration last month, accusing
the organisation of slowing down their inquiry.
The 10-member, bipartisan panel has until May to report on its
investigation into lapses in national security relating to the plane
hijackings that led to the deaths of about 3,000 people.
They had requested information from the North American Aerospace Defense
Command, Norad, which is responsible for protecting North American
airspace.
Ministerial backing
"The commission has encountered some serious delays in obtaining needed
documents from the Department of Defense," they said in a statement.
"We are especially dismayed by problems in the production of the records
of activities of ... Norad and certain Air Force commands on September
11, 2001."
"The commission has therefore voted to issue a subpoena requiring the
production of these records."
They issued a subpoena to the Federal Aviation Administration in October
requesting information on air traffic control tracking of hijacked
aircraft.
Defence Secretary Donald Rumsfeld was said to back the commission's
request for documentation.
"The commission has a statutory deadline to meet," said Defense
Undersecretary for Intelligence, Stephen Cambone.
"The Secretary has directed that the department be responsive to held
ensure the commission can meet its deadlines."
http://news.bbc.co.uk/2/hi/americas/3253401.stm
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