Dusseldorf - Police in the western German state of North Rhine Westphalia have been searching for Norman Franz, who is suspected of killing five people, for the past 20 years.

After issuing a public call for information on a TV show last week, the state's criminal police office has received more than 50 tip-offs from members of the public, the police department said on Thursday. The police say they will look into all potential leads.

The State Criminal Police Office of North Rhine Westphalia issued a call for information on a monthly show that aims to solve crimes.

The 48-year-old suspect, who is one of Germany's most wanted criminals, disappeared nearly 20 years ago.

The investigators presented a voice sample from a recorded telephone call on the show, seeing as police have no current photographs of Franz.

Franz was sentenced to life imprisonment in March 1995 after killing two people by throwing a hand grenade into their car. On March 11, 1997, he escaped from prison in the western city Hagen.

That same year, he allegedly shot three security guards in Weimar and in Halle during armed robberies. Then, he fled the country to Portugal with his wife, where he was apprehended on October 24, 1998.

But before he could be extradited to Germany, he was once again able to escape from prison in Lisbon: On July 28, 1999, Franz escaped by sawing through the bars on the window of his cell, lowering himself out of the window with the bedsheets and then climbing over four walls.

Police are offering a 25 000-euro (31 000-dollar) reward.