* Investigation far advanced into who was involved
* Scores of Americans could have been killed
By James Vicini and Jeremy Pelofsky
WASHINGTON, Oct 6 (Reuters) - A recently disrupted bombing plot represented one of the most serious security threats to the United States since the Sept. 11 attacks, Attorney General Eric Holder said on Tuesday.
"I can say the investigation is pretty far along. We have a pretty good handle who was involved and what was intended," Holder told a news briefing.
"This alleged plot was one of the most serious terrorist threats to our country since September 11, 2001."
An Afghan immigrant, Najibullah Zazi, was indicted last month by a federal grand jury in New York on charges of plotting to explode bombs in the United States. Zazi, who is being held without bail, has pleaded not guilty.
Prosecutors said Zazi took a bomb-making course at an al Qaeda training camp in Pakistan, had notes on how to make explosives his laptop computer and acquired materials similar to those used in bomb attacks in London in 2005, buying acetone and hydrogen peroxide at beauty supply stores.
Holder said the plot, if it had been successful, could have killed "scores" of Americans, based on the chemicals involved, the history of similar plots and the number of people suspected of being involved.
The FBI has had under surveillance other suspects it believes may have helped Zazi acquire the chemicals and consulted with him on how to make explosives.
Asked about whether there would be more arrests and others charged, Holder replied, "It is our intention to bring all those involved in the plot to justice" and "the investigation is ongoing."
Zazi initially was charged in Colorado with lying to the FBI. He was later indicted on the terrorism charge in Brooklyn federal court and transferred to New York on Sept. 25 to face that charge.
The investigation became public several weeks ago when police raided apartments in the New York City borough of Queens that Zazi had visited around the time of the anniversary of the 2001 attacks.
AL QAEDA CONNECTION
Holder said Zazi visited Pakistan in 2008, when he allegedly attended an al Qaeda training camp. "There certainly was an al Qaeda connection," Holder said.
Zazi’s attorney has insisted that his client traveled to Pakistan for innocuous reasons -- to see a dying relative, to get married and to visit his wife.
The suspect’s father, Mohammed Zazi, and a New York imam, Ahmad Afzali, accused of having tipped off the younger Zazi that he was under scrutiny, also were arrested on charges making false statements to the FBI.
The New York Police Department had used the imam in the past as an informant. When they asked him about Zazi, he alerted Zazi that he was under surveillance, forcing federal officials to bring him in for questioning sooner than they had initially planned.
"Are there things we could have done differently? I‘m sure we’ll find there are," Holder said, adding that overall he was pleased at how well the New York police worked with the FBI in the case.
Holder said information obtained from Foreign Intelligence Surveillance Act warrants helped unravel of the plot.
He also cited the investigation in urging Congress to extend the three surveillance techniques in the Patriot Act that expire later this year, calling them vital tools in protecting the country. The law first was adopted by Congress during George W. Bush’s presidency after the 2001 attacks. (Editing by Chris Wilson)