Prescription drugsThe mysterious $500 million Google set aside in the last quarter for an ongoing Department of Justice probe was finally made clear last night. Google is apparently close to settling a criminal investigation over ads from illegal online pharmacies that have earned the company hundreds of millions, the Wall Street Journal reports.

The half a billion dollar fine would be among the largest paid to the government to settle investigations, the Wall Street Journal notes, and it’s a sign that Google is willing to pay whatever it takes to make this investigation go away. It’s a wake up call to the company, which generated nearly $30 billion in ad revenue last year, that it needs to be particularly careful with the ads it accepts.

In 2007, Google, Yahoo, and Microsoft paid a combined $31.5 million fine over accepting ads from illegal online gambling sites. Search engines need to be particularly careful about where their ads come from, as they can be held liable for making money from illegal sites.

The investigation is being led by the U.S. Attorney’s Office in Rhode Island and the FDA, as well as a few other agencies, sources tell the WSJ.

The probe is part of an ongoing battle Google has had with illegal online pharmacies. The company banned ads in 2003 that offered access to drugs like Vicodin and Viagra without a subscription, but in 2004 it said that it would continue to carry ads from Canadian pharmacies that send drugs to U.S. customers. That attitude changed last year when the federal investigation began, with Google saying in February that it would only allow ads from accredited U.S. pharmacies and legitimate Canadian pharmacies.

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