Agent Harry Samit took the witness stand at the death penalty trial of the confessed Al-Qaeda plotter.
Agent Samit, who arrested Moussaoui, also admitted under cross-examination that he accused his superiors of "criminal negligence" and "careerism" in a US government inquiry into their failure to act upon his warnings.
The FBI agent was shown a memo he sent to FBI headquarters in Washington on August 18, 2001, parts of which were sent across the federal government, in which he referred to Moussaoui 70 times.
"You needed people in Washington to help you out?" Defence Counsel Edward Macmahon asked Agent Samit. "They didn't do that, did they?" Agent Samit answered: "No."
In the document, Agent Samit warned the 37-year-old Frenchman was learning how to steer a 747-400 airliner, had a portable GPS navigation system, was an Islamic fundamentalist who approved of matyrdom, and was armed with small knives and learning martial arts.
Moussaoui is the only man tried in the United States in connection with the September 11 attacks, which killed nearly 3,000 people.
Hurry to finish
Agent Samit also said Moussaoui seemed to be in a hurry to finish his flight training on a jumbo jet simulator in Minnesota when he was arrested.
"You wanted the people in Washington to know that because you were concerned that Moussaoui was going to try to hijack a plane right?" defence lawyer Edward MacMahon asked Agent Samit. "Yes sir," the agent replied.
Agent Samit also said he pleaded with his supervisors to warn the US Secret Service, which protects the president, after learning Moussaoui had planned trips to New York and to look at the White House in Washington.
That revelation was especially alarming, because he discovered Moussaoui had told his flight simulator instructor to show him how to complete a flight between London Airport, and New York.
"If he seized an airliner flying from Heathrow to New York City, it would have enough fuel onboard to reach D.C.," he said he told a supervisor in Washington, but got no word that the warning was ever passed on.
Agent Samit also said superiors in FBI anti-terrorism units in Washington had stopped him from applying for a criminal search warrant or to a special intelligence court, so he could search Moussaoui's belongings.
Prosecutors want to prove he deserves death, because his "lies" to investigators allowed the suicide hijackers time to carry out the strikes on New York and Washington.
Defence lawyers say the government knew far more than Moussaoui about the looming threat, and failed to act to stop it.
If he is not sentenced to death, Moussaoui, in jail on September 11, 2001, he will spend the rest of his life in prison, after confessing in April last year to conspiring to fly airliners into prominent US buildings.