BERLIN (AFP) - Police searching a home of the German co-pilot who officials say appears to have deliberately crashed a plane into the French Alps said on Friday they had seized evidence, but no "smoking gun".

Officers combing through a flat kept by Andreas Lubitz in the western city of Duesseldorf said they had seized "various items and papers", police spokesman Marcel Fiebig told AFP.

"We will see whether this will explain what happened - everything is being examined," he said, adding that there was no "smoking gun" to shed light on a possible motive.

Duesseldorf prosecutors have opened a parallel inquiry to the main investigation underway in France as many among the 150 crash victims were from the German region.

Lubitz, who the local authorities say was 27, spent most of his time at his parents' home in the small western town of Montabaur.

That upscale residence on a quiet leafy street was also cordoned off by the police on Thursday and searched as camera teams massed outside.

Men wearing gloves came out carrying briefcases, bags and boxes, an AFP journalist reported.

The city's public prosecutor said in a written statement that searches in Duesseldorf and other places were aimed at "the discovery and securing of personal documents" to help clarify the situation.