Brazilian authorities considered shooting down the plane, which had no flight plan and was viewed as a threat, according to the March 28, 2009, report. Air defense authorities followed the country's shootdown procedure, illustrating "the extreme caution with which a possible shootdown is approached, the broad understanding of the shootdown policy among air traffic controllers and the fact that the procedures are executed as written," Sobel said.

However, the procedure wasn't fast enough: The plane went down while the decision went up the chain of command.

"The deliberate approach to a shootdown decision highlighted a vulnerability to potential terrorist actions given that a decision would not have been made in time to stop the pilot had he been able to crash into his target or another building, including in Brasilia," Sobel said in the memo.

He said the vulnerability was due in part to the fact that a 2004 law that allows Brazilian air force planes to shoot down small aircraft was aimed at curbing drug smuggling not terrorist attacks.

The law seeks to seal off the Amazon jungle route to drug traffickers. Most of the cocaine trafficked through Brazil comes through the border region from neighboring Colombia, Bolivia and Peru.

Under the law, a long list of precautionary steps must be taken before the shootdown order is given.

Air force pilots must make visual contact with a suspected plane, try to make radio contact, try to change the plane's route and fire warning shots before seeking authorization to shoot the aircraft.