The Wall Street Journal reported yesterday that more than 70 people have been arrested in the U.S. and Nigeria in recent months and accused of participating in a sophisticated email scam that has targeted American businesses and cost them hundreds of millions of dollars this year alone.

In a sweep dubbed Operation Wire Wire, the FBI targeted an alleged West African organized crime ring and others who sent fake messages to companies’ finance departments purporting to be a vendor for the company with an invoice requiring payment.

The number of complaints to the FBI’s internet crime center about so-called “business email compromise” scams has shot up in recent years. In 2015, the center said it received 8,000 complaints by either businesses or individuals who reported that money was mistakenly wired in response to fraudulent emails asking for payment.

Victims reported $275 million in losses in 2015, the FBI said, and that had more than doubled by 2017 to $675 million.

Read more from the Wall Street Journal.