The four men accused of plotting to blow up two New York synagogues and shoot down military planes could have procured surface-to-air guided missiles elsewhere if the FBI had not provided them with inert versions, terror analysts told FOXNews.com.

"I don't know if you could buy it on Craigslist, but there's certainly a lot of people who engage in this type of contraband," Steve Emerson, executive director of The Investigative Project on Terrorism, said of the anti-aircraft Stinger missile. "They're not that big, either, so they could've been smuggled into the United States."

Emerson said the 5-foot-long weapon, which has a range of 5 miles and weighs 35 pounds fully armed, could have been bought in a number of black arms markets in Middle Eastern countries, including Lebanon, Pakistan, Gaza and Saudi Arabia. The missile system could be purchased for "tens of thousands of dollars," Emerson said.

"It depends on the quality, how new it is, the version," he said. "It's like buying a used car. It depends on lots of factors, and they deteriorate over time."

The four domestic terror suspects — James Cromitie, David Williams, Onta Williams and Laguerre Payen, all of Newburgh, N.Y. — were arrested late Wednesday after they allegedly planted a 37-pound device that they believed was a bomb in the trunk of a car outside the Riverdale Temple, a synagogue in the Bronx, and two other mock bombs in the backseat of a car outside the Riverdale Jewish Center, another synagogue a few blocks away. They also allegedly planned to shoot Stinger surface-to-air guided missiles at planes at the Air National Guard base in Newburgh, about 70 miles north of New York City.

FBI investigators had been monitoring the men and, through an informant, provided them with an inactive missile and inert C-4 explosives, according to the federal complaint filed against the suspects.

Emerson said domestic terrorists could smuggle authentic weapons into the United States by ship, using an uncontrolled segment of shoreline as a point of entry, possibly along the Florida coast. Once in the U.S., not too many hurdles would remain, he said.

"A Stinger missile is pretty easy to operate and it's very portable," Emerson said. "A Stinger doesn't take a lot of training at all, and you can do a lot of damage."

Acquiring a Stinger in the United States would be "much more difficult" but not impossible, Emerson added.

"There's always leakage," he said, adding that "there used to be a lot floating around" following the Soviet war in Afghanistan in the 1980s. "Terrorists are always after these types of weapons," he said.

The CIA reportedly supplied some 500 Stingers to the Mujahideen fighting the Soviet forces, and the U.S. tried to reclaim them through a buy-back program after the war.

But an unspecified number remain unaccounted for, said Lawrence Korb, a senior fellow at American Progress, a Washington-based think tank.

"We were aiding the Mujahideen and now you're living the consequences," Korb said. "The CIA calls it blowback."

Korb said it's "absolutely possible" that the terror suspects could have believed they were purchasing one of the weapons left behind decades ago.

"These people know they're out there," he told FOXNews.com. "If you have the money, they're out there."

Meanwhile, Michael Barkun, professor of political science at Syracuse University, said it's apparent the four suspects did not have the connections needed to carry out such an attack.

"Obviously they had no access to those markets or they would've gotten them," Barkun said. "It's clear these are bad guys, but they appeared to have no other access to the weapons other than those people who they believed were their suppliers.

"They had the intent, but did not have the means," Barkun continued. "They thought they had the means. Thank God they did not."