BERLIN--German prosecutors on Thursday raided the offices of Volkswagen AG to secure evidence in their investigation of the company in connection with a widening emissions-cheating scandal.

The prosecutor's office in Braunschweig, which is overseeing the criminal probe, said three state attorneys and 50 state police officers raided Volkswagen's headquarters, as well as private homes, in Wolfsburg and other locations. A spokeswoman said the initial investigation into probable causes is focusing on "several people."

"In light of potential crimes, the aim of the searches was to secure documents and data storage devices that could provide information about employees of the company and their identities involved in the manipulation of emissions of diesel automobiles," the prosecutor said.

The prosecutor gave no further details.

Volkswagen initiated the investigation on Sept. 23 when it filed a complaint with the prosecutor's office requesting a probe into the emissions cheating affair. The company said Thursday that it had turned over to prosecutors a "comprehensive collection of data" at its headquarters in Wolfsburg.

"We will assist the prosecutors in their investigation of this matter and the responsible people to the best of our ability. This serves the rapid and complete solution, in which Volkswagen has great interest," the company said.

The move comes after the disclosure last month that Volkswagen had installed software on as many as 11 million diesel-powered cars that allow t he vehicles to manipulate emissions tests. The affair has rocked Europe's biggest car maker, forcing its former chief executive to resign under pressure, and raises broader questions about the auto industry's compliance with emissions regulations.

The scandal that has come to be known as "diesel gate" was set off in the U.S. when the country's environment authorities disclosed on Sept. 18 that Volkswagen installed the so-called "defeat device" on nearly 500,000 cars in the U.S.

The head of Volkswagen's American business, Michael Horn, is set to testify about the matter before a congressional committee later Thursday. U.S. lawmakers are expected to grill him about how and why Volkswagen tweaked diesel-powered cars to cheat emissions tests.

Volkswagen AG is also under investigation in Europe. The company presented a comprehensive plan to repair cars with tainted diesel engines, German authorities said Thursday, but it remains unclear whether the software was used to cheat emissions tests in Europe.

Mr. Horn has been up front with U.S. authorities, but back in Europe, where one in four cars sold come from the Volkswagen group, regulators, consumers and car dealers are still waiting for clarity on a key question in the crisis: Did Volkswagen also cheat on European tests?

In response to a query by The Wall Street Journal, Volkswagen issued a less than definitive response, saying that the "software built into some of these diesel-powered cars can, in theory, detect when they are being tested and influence emissions."

"Whether this software was actually engaged and to what extent is still the subject of internal and external investigations," Volkswagen said in an emailed response to Journal queries. "It is also still legally unclear whether this is an illegal defeat device in the purview of European standards."

Volkswagen underscored that the only diesel engine to suppress emissions in the laboratory, but then turn off this device on the road is the EA 189, a four-cylinder TDI diesel engine that comes in 2.0 liter, 1.6 liter and 1.2 liter versions.

The software installed on the engine recognizes when a car is being tested in the lab and neutralizes nitrogen oxide emissions to meet environmental standards. But once outside the lab, the software allows higher nitrogen oxide emissions to allow better performance and mileage.

Volkswagen Chief Executive Matthias Müller said this week that in January the company would launch a global recall of vehicles affected by the software. The total of 11 million vehicles spreads across several Volkswagen brands, including VW, Audi, Skoda and SEAT.

The largest number of vehicles is in Europe, where eight million are affected, nearly three million of those in Germany and more than a million in the U.K., Europe's second-largest car market after Germany.

Some of the cars require a quick fix, a simple software update, while others would require switching out the fuel-injection nozzles. The entire recall is expected to be completed by the end of 2016.

Sigmar Gabriel, German economy minister and vice chancellor, warned Volkswagen to be completely open about the affair and quickly repair the damage.

"My only advice is to clear this up very quickly," Mr. Gabriel, leader of the left-leaning Social Democrats, said Thursday in Wolfsburg, after a meeting with VW's top labor representatives. "The company must disclose what happened. The more aggressively it does this, the better it will be."

Making cars is Germany's biggest single industry and central to its economy, accounting for one in seven jobs. Against this backdrop, Mr. Gabriel said the industry as a whole shouldn't suffer on account of Volkswagen's actions without proof that the misconduct is more widespread.

Write to William Boston at william.boston@wsj.com, Ruth Bender at Ruth.Bender@wsj.com and Stefan Lange at stefan.lange@wsj.com

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