WASHINGTON (AP) — The U.S. Drug Enforcement Administration seized more than $36 million in cash and arrested 91 people in a nationwide crackdown against manufacturers, distributors and vendors of synthetic designer drugs.

The DEA administrator, Michele Leonhart, said Thursday agents in 31 states also seized 4.9 million packets of synthetic marijuana, material to make 13.6 million more packages and 167,000 packages of synthetic hallucinogens known as bath salts during a series of raids on Wednesday. DEA and other law enforcement agencies also seized materials to make 392,000 more packets of bath salts.

Leonhart said the synthetic drugs are “marketed directly to teenagers.”

“Many of these products come with a disclaimer that they are ‘not for human consumption’ to mask the danger they pose,” Leonhart said.

The agents raided smoke shops and other sellers of synthetic marijuana and other synthetic drugs that have been linked to psychotic episodes and deaths of users. Leonhart said agents shut down 29 manufacturing sites that ranged from massive warehouses to residential basements. She said many of those arrested Wednesday had extensive criminal histories.

“But we also found a number of people that are new to the drug business and have been able to make connections with some of these more seasoned traffickers,” Leonhart said.

Those arrested could face a variety of state or federal criminal charges, she said.

The drugs have become a popular alternative to traditional street drugs, but law enforcement and health professionals have warned that the chemicals used to make the synthetic marijuana and hallucinogenic bath salts haven’t been tested or approved for human consumption. The synthetic marijuana is sold under brand names such as “K2″ and “Spice.”

The agency temporarily has banned some of the chemicals found in synthetic marijuana, and President Barack Obama this month signed into law a measure that bans the sale, production and possession of many of the chemicals found in the most popular synthetic drugs.

But experts who have studied the drugs estimate that there are more than 100 different bath-salt chemicals circulating. Bath salts can mimic the effects of cocaine, LSD and methamphetamine.

Use of the drugs has grown since the synthetic products first hit the market a few years ago. They are readily available for purchase in smoke shops and sometimes even corner gas stations, and at a relatively low price, and that has made them a popular alternative to street drugs.

As the drugs have become more popular, side effects have become evident to health professionals. Doctors and police have struggled at times to control bath salt users who often become feverish and paranoid that they are being attacked. Several deaths have been attributed to the drugs. ___

Associated Press writer Matthew Perrone contributed to this report.

Copyright 2012 Associated Press