"There are a couple of key concerns here that make it a little more unusual than the norm including the printer bomb plot," said Borelli.
The DHS memo cites a range of possible attack options by terrorists. "Based on our review of historical Homeland and overseas terrorist plots, we assess that terrorists plotting an attack within the United States could use IEDs placed and abandoned at predetermined locations or carried or worn by suicide operatives, remotely detonated multiple IEDs that are timed for sequential explosions, or VBIEDs [car bombs].Terrorists could also use tactics featuring small arms to conduct an attack, possibly in combination with IEDs."
New York City police conducted a drill to prepare for a Mumbai-style small arms attack earlier this year.
Worries about vehicle-borne threats were brought up in three earlier DHS briefing memos sent to law enforcement that are circulating with the holidays approaching. The bulletins warn of possible attacks from tractor trailers and buses, the use of vehicles to breach security perimeters, and "Terrorist Use of Vehicle Ramming Tactics."
"The ramming warning likely comes from an online Al Qaeda magazine, which advocated attaching blades to a pickup and driving into a crowd in Washington, D.C.," said Borelli, now a senior v.p. of the Soufan Group, a security consulting firm. The tactic was recommended in a summer issue of Inspire, Al Qaeda in the Arabian Peninsula's webzine.
The most recent bulletin to mention vehicle ramming, a "Roll Call Release" dated this Monday, says the tactic "offers terrorists with limited access to explosives or weapons an opportunity to conduct a Homeland attack with minimal prior training or experience."
An earlier Roll Call Release dated June 10 and now recirculated, states, "Terrorists could use large commercial motor vehicles ... such as trucks or buses loaded with explosives or toxic industrial chemicals as weapons to attack public gatherings; secure or sensitive sites such as airports, power facilities, critical bridges and tunnels, or fuel depots. A terrorist also could use a CMV as a battering weapon."
And a third release also dated June 10 states, "Analysis of attacks by violent extremist groups overseas reveals that attackers seek ways to circumvent or overcome fencing, access road barriers, and similar perimeter security measures to gain access to their primary target. Law enforcement and security personnel should be aware that individuals seeking to conduct an attack in the United States may attempt unorthodox or unexpected measures, such as using improvised ramps, to bypass obstacles."
Combined, the circulars demonstrate the level of concern about what top officials have already acknowledged is a significant uptick in the terrorists' operational tempo.
Said Cilluffo, "The jihadi threat we face today has metastasized and comes in various shapes, sizes, flavors and forms, ranging from Al Qaeda senior leadership to its affiliates in [Pakistan's] Tribal Areas and Waziristan, Yemen , Somalia, the Maghreb and increasingly the Caucasus, to homegrown jihadists. "
The DHS holiday bulletin also reminds readers, however, that "the timing of a terrorist attack depends more on terrorists' readiness to execute" it than on the calendar.