Feb. 23, 2010 -- Najibullah Zazi's foiled plot to attack New York City subways late last year is the latest example that the fight against al Qaeda has entered a dangerous new phase, federal law enforcement officials say.
In the span of three months, al Qaeda launched two separate, well-orchestrated attacks against the U.S. homeland that could have been devastating had they not been stopped.
"This attack was real, it was in motion and it would have been deadly," Attorney General Eric Holder said of the Zazi plot at a press conference Monday.
The same could be said about the al Qaeda-trained Umar Abdulmutallab's plot to blow up Northwest flight 253 on Christmas Day.
Zazi, a Queens, N.Y.-raised Afghan, told a New York federal court yesterday he conspired to detonate a bomb and was just days from putting his plot into motion around the eighth anniversary of September 11.
"I would sacrifice myself to bring attention to what the U.S. military was doing to civilians in Afghanistan," he said.
Zazi pleaded guilty to charges of conspiracy to use weapons of mass destruction, commit murder and provide material support to al Qaeda.
"When you see the charges, these guys, this was not an amateur operation," said New York City Mayor Michael Bloomberg at a press briefing. "It was a very serious plot against New York City, and fortunately, the great work by the FBI and the NYPD together stopped this plot. Let us just hope they stop the next one."
Senior law enforcement officials told ABC News that al Qaeda has apparently decided that it will no longer solely pursue attacks on the scale of 9/11 that kill thousands.
Instead, sources say, the group has made a strategic decision to pursue smaller-scale attacks inside the U.S. to achieve at least some tactical success.
"They want to see bodies, blood sprayed all over the place. They want to punish us. This is absolutely what they want to do. Make no bones about it," said former FBI agent Jack Cloonan. "It accomplishes a number of things aside from body count. It reaffirms that they are alive and well."
Expert: Al Qaeda Has 'New Cadre of People' to Launch Attacks
National security officials remain on high alert because of what they say is a steady stream of intelligence pointing to al Qaeda actively planning more attacks.
Federal agencies have received intercepts of Islamic radicals bragging that something is coming and informants warning of the same.
"They've reconstituted. They have a new cadre of people. They're younger, just as motivated," Cloonan said of al Qaeda.
U.S. officials have openly predicted another attempted attack is coming soon, within the next three to six months.
"What is the likelihood of another attempted terrorist attack on the US homeland in the next 3-6 months? High or low?" asked Sen. Diane Feinstein, D-Calif., at a hearing by the Senate Select Intelligence Committee on Feb. 2.
"An attempted attack, the priority is certain," replied Director of National Intelligence Dennis Blair.
Blair's colleagues, CIA director Leon Panetta and FBI director Robert Mueller, agreed.
ABC News' Devin Dwyer contributed to this report.