Federal prosecutors in Manhattan Wednesday announced the biggest case to date on the latest drug scourge to hit the metropolitan area, charging 10 men with running a ring that sold more than a quarter million retail packets of so-called synthetic marijuana, a popular drug marketed in bodegas.

Manhattan U.S. Attorney Preet Bharara called the case the start of "an aggressive assault on a dangerous new public health crisis reaching epidemic proportions." He said despite the name and the fact that they are not covered by state criminal laws, the drugs are "poison in a packet."

"This is not marijuana," Bharara said at a morning news conference attended by NYPD Commissioner William Bratton. "They are packaged like children's candy, but they are lethal."

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"The risk is incredible," Bratton said.

Synthetic cannabinoids -- known on the street as "spice" or "K2" -- are chemical compounds sprayed with fruity flavorings onto leafy substances like tea. They are sold in small groceries in $5 packets with eye-catching wrappers and names like "Scooby Snax," and "Green Giant."

The drug isn't classified as a controlled substance in New York. The low cost has led to growing use by the young, the homeless and prisoners and causes rapid heart rate, kidney damage and paranoia, officials said, adding that the mentally ill are especially vulnerable.

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The 10 men charged, identified as Yemeni-Americans, were accused of importing more than 100 kilograms of illegal synthetic compounds from China in the past year, and running processing facilities and warehouses to mix and spray them onto leaves, and bundle and store retail packets.

Some defendants allegedly served as wholesalers who bought packets in bulk and distributed them to more than 70 retail locations in the five boroughs. Others delivered the merchandise. One retailer was charged. In warehouse raids Wednesday, authorities said, they seized chemicals and processed packets with a potential street value of $30 million.

The indictment said in one recent two-month period, use of the drug resulted in 2,300 emergency room visits in New York State -- a tenfold increase over 2014, Bharara said. Nationally, calls to poison centers increased 229 percent this year over last, Bharara said.

In the absence of criminal sanctions, the city has tried to use public health provisions to prevent bodegas from selling the drug. Bharara called the case a "warning" to bodega owners who may have thought synthetic marijuana is legal that they could face criminal prosecution in the future.

"Until now, these poisons have been sold too openly and without fear of prosecution," Bharara said.

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Six of the defendants were arrested Wednesday and presented in court. They each face up to 20 years in prison for conspiracy to distribute narcotics.