German police seeking evidence of who supplied guns and explosives to a neo-Nazi gang of racist killers raided homes and shops of four new suspects on Wednesday, prosecutors in Karlsruhe said.

They gave no word of arrests, but Federal Prosecutor General Harald Range said the four new names increased the list of suspected supporters of the self-styled National Socialist Underground (NSU) to 11, DPA reported.

All 11 extremists are believed to have quietly aided the trio of neo-Nazis who adopted fake identitities in a seven-year killing spree of immigrants between 2000 and 2007. The victims comprised eight Turkish shopkeepers, a Greek and German policewoman.

The killings remained unsolved until the gang imploded two months ago after a failed bank raid, with a joint suicide by two and the survivor turning herself in.

Most of the suspects are free or on bail, but the survivor, Beate Zschaepe, 36, and four of the alleged supporters remain in custody.

"One of our most urgent tasks is to identify the complete circle of supporters and bring them to justice. In recent weeks we have made progress towards that," Range told reporters.

The raids were conducted by 110 police in three states. Two of the alleged supporters newly under investigation are suspected of supplying an explosive and a gun to the group in 1998, and two others were believed to have provided guns in 2002 and 2003.

Their names were not released, but the daily Westdeutsche Allgemeine Zeitung said one of the suspects had been an official of the youth wing of the far-right National Democratic Party (NPD) and had actively helped the trio from 1999 and 2000 before quarrelling with them.

There are plans for a national commemoration next month of the victims. Germany's parliament is expected to appoint a commission of inquiry this week to establish why the nation's 34 separate police and domestic intelligence agencies failed to spot the trio.

Critics say turf wars between the agencies were partly to blame. Police perceptions that the victims were most likely to have been killed by fellow immigrants have triggered charges of police racism.

Zschaepe is expected to face arson charges for blowing up the trio's home in the city of Zwickau, apparently to destroy evidence.