The pharmaceutical company AstraZeneca said Thursday that it had reached a $520 million agreement to settle two federal investigations and two whistle-blower lawsuits over the sale and marketing of its blockbuster psychiatric drug Seroquel.

One of the investigations related to “selected physicians who participated in clinical trials involving Seroquel,” AstraZeneca disclosed in a government filing. The other case related to off-label promotion of the drug.

As a result of aggressive marketing, Seroquel has been increasingly used for children and elderly people for indications not approved by the Food and Drug Administration. Doctors are permitted to prescribe any approved drug for off-label uses.

Seroquel was the top-selling antipsychotic drug in America. It had $17 billion in sales in the United States since 2004, according to IMS Health, a research firm.

Tony Jewell, a company spokesman, declined to be more specific about the physicians or clinical trials under investigation. He said the company was in final negotiations to settle the whistle-blower suits and reach a corporate integrity agreement with the Justice Department.

The name of the whistle-blowers and other details of the suits remained sealed in federal court. Stephen A. Sheller, a lawyer in Philadelphia for the whistle-blowers, and Patricia Hartman, a spokeswoman for the United States attorney in Philadelphia, both declined to comment.

AstraZeneca, based in Britain, joins a list of drug makers that have paid billions to settle inquiries initiated by complaints from former company insiders.

Earlier this year, Eli Lilly & Company paid $1.4 billion over its marketing of Zyprexa, another antipsychotic drug. And Pfizer announced it would pay $2.3 billion, including a record $1.195 billion criminal fine, mostly over its painkiller Bextra, which has been withdrawn from the market.

AstraZeneca disclosed the settlement in a financial report. Third-quarter revenue rose 10 percent, to $8.2 billion, and operating profit rose 29 percent, to $3.6 billion, at constant exchange rates over the year-earlier quarter.

AstraZeneca also said it had been served with 14,444 civil lawsuits over the drug as of Oct. 9. Ed Blizzard, a lawyer for some of the people suing AstraZeneca, said Thursday that many patients have developed diabetes and other health problems because of misleading marketing.

Mr. Blizzard said he did not know what clinical trials were part of the inquiry. But in one trial, known as Study 15, he noted, an e-mail message showed a company official saying “a great ‘smoke and mirrors’ job’ ” had been done on a “buried” study in 1997, the year the F.D.A. approved Seroquel.