The Drug Enforcement Administration and other federal agencies served hundreds of search and arrest warrants across the United States on Wednesday in what officials described as the largest ever crackdown on people who make and distribute synthetic designer drugs. James Capra, the D.E.A. chief of operations, said bank accounts were seized in 35 states, as well as more than 1,000 kilograms of drugs used in synthetic marijuana, bath salts and other substances that can mimic cocaine, LSD and other drugs. The drugs are often marketed as herbal incense or other seemingly innocuous products.