WASHINGTON — Federal drug agents have arrested hundreds of people across the country in the latest phase of a national crackdown on synthetic drug manufacturers, wholesalers and dealers, a federal official said Thursday.

The official said the Drug Enforcement Administration and other state and federal agents arrested several hundred people in more than 20 states over the past 15 months in an operation that ended Thursday.

Agents in Los Angeles and Bakersfield, California, seized about $500,000 in cash and more than 200 pounds of drugs.

The DEA has been cracking down on synthetic drugs, including bath salts, Spice and Molly, since the drugs gained widespread popularity years ago.

In late 2010, the agency responsible for enforcing federal drug laws moved to ban five chemicals used to make synthetic marijuana blends, including K2, Spice and Blaze. Since then, drug manufacturers have continued to modify their formulas and develop new chemical mixtures.