Over the last three years, he said, the amount of Colombian heroin seized by United States agents has doubled every year.

Although Colombian heroin represents 30 percent of all heroin seizures by the drug agency nationwide, its market share is believed to be considerably less.

Their market growth, however, worries officials because Colombian heroin is not only cheaper, but also purer, than the old Southeast Asian variety.

"About four years ago, we didn't see any Colombian heroin," Mr. Van Etten said.

Some law enforcement officials believe that the Cali cocaine syndicate is behind the influx of Colombian heroin.

Rick Curtis, a sociologist who interviews both dealers and addicts on the street, said, "From what we've heard there is a lot more Colombian heroin.

After steadily expanding its market in recent years, white Colombian heroin now dominates east of the Mississippi; brown Mexican heroin rules to the west.

Couriers carrying Colombian heroin commonly hopscotch Central American airports to Mexico.

Colombian heroin will rise by as much as 20 times once it gets to Los Angeles.

The flow of Colombian heroin is also increasingly routed through Nigeria, drug agents said.