Federal agents arrested more than 90 people and seized about 5 million packets of synthetic drugs - known on the street as "bath salts," "K2" and "Spice" - in the first U.S. nationwide crackdown on "designer drugs," the Drug Enforcement Administration said on Thursday.

The DEA said the haul from multi-agency raids in more than 100 U.S. cities included 4.8 million packets of synthetic cannabis, 167,000 packets of "bath salts" and more than $36 million in cash.

Operation Log Jam, which involved the DEA, five federal agencies, state and local police, "has disrupted the entire illegal industry, from manufacturers to retailers," DEA Administrator Michele Leonhart said in a statement.

"Together with our federal, state and local law enforcement partners, we are committed to targeting these new and emerging drugs with every scientific, legislative and investigative tool at our disposal," she said.

Synthetic drugs - with street names like "bath salts" or "plant food" - mimic more traditional illegal narcotics like cocaine, LSD, MDMA and methamphetamine, and have rapidly grown in popularity, the DEA said.

Bath salts have been blamed in several incidents of erratic and violent behavior around the country.

Smokable herbal blends marketed as "Spice," "K2," "Blaze," and "Red X Dawn" and labeled as incense contain illegal psychoactive compounds that mimic THC, the active ingredient in marijuana. They have become popular among teens and young adults, the DEA statement said.