Ask any law enforcement official and they will tell you no day is off limits to terrorists. That's really the point, the victims don't know what's coming or when. The Twin Towers were attacked on a Tuesday back in 2001. A day of no particular significance.

As New York prepares for the 10-year anniversary of the September 11th attacks, the date holds far deeper meaning. A symbol of vulnerability, the deadliest act of terrorism committed on U.S. soil will be observed Sunday, in what will likely be the most significant anniversary of 9/11 for some time.

The Intelligence: Surveillance of al-Qaeda has resulted in intelligence indicating a desire to carry out an attack this Sunday, which would disrupt the observance, and serve as a symbol that the terrorist organization is still operable.

When Osama bin Laden was killed on May 1, 2011, American forces seized documents from his compound in Abbottabad, Pakistan. Analysts found notes planning for an attack on the 10th anniversary, including a potential attack on the rail system. Impossible to fully guard, public transportation systems have often been the target of al-Qaeda operatives over the last decade.

"Al-Qaeda remains a seminal threat to New York City and the homeland," said Deputy Chief Michael Blake of the NYPD's Counterterrorism Division at a police department briefing Wednesday.

Describing a threat that has not diminished with the death of bin Laden. Several figures are of interest to the intelligence community, operating in the Afghanistan/Pakistan theatre including Mullag Omar, Jalaluddin Haqqani and Gulbuddin Hekmatyar. In Pakistan, an umbrella organization has even emerged for different Pakistani factions called, Tehrik-e Taliban Pakistan.

While the TTP largely targets Pakistanis including the assassination of Benazir Bhutto in 2007, it has taken an international targets of late - implicated in the Barcelona Plot of 2008 and the bombing attempt on Times Square in 2010.

Law enforcement officials believe al-Qaeda is, without question, weaker on the 10th anniversary than it was in 2001. Its weakened state may explain the evolution of terrorism in our country. The 1993 and 2001 attacks on the World Trade Center along with the 2000 attack on the USS Cole were complicated strikes requiring cells to operate with significant amount of training and planning. The modern day threat, dubbed "lone wolves," involve individuals operating largely on their own. Najibullah Zazi, Umar Farouk Abdulmutallab and Faisal Shazhad operated without the traditional hierarchy the al-Qaeda network - allowing them to operate under the radar of anti-terrorism agencies.

The NYPD has outlined a large area surrounding the World Trade Center site that will be closed off in what's called a "frozen zone" Sunday. President Barack Obama and former President George Bush Jr. are amongst those who will attend.

Thousands of extra uniformed officers will be brought in including snipers, bomb squad personnel and hostage negotiators. Last week, the FBI issued a warning of potential threats from access to small aircrafts, tho no specific plot has been identified.

The police department's resources will be stretched out with several high profile event, all planned for the same day. The US Open will be in progress with the NY Mets playing across the way, in addition to an NFL game and the continuation of fashion week.