Police in Germany have launched an investigation and stepped up their search for more explosives after at least two more incendiary devices were found Wednesday in the Berlin railway system.

It is the third straight day police have discovered devices filled with flammable liquid near train stations or rail lines.

One of the devices had caught fire when Wednesday, and a similar device was used in a separate arson attack on Monday along a high-speed link to the northern city of Hamburg. The Monday attack caused enough damage to temporarily stop train service, leading to major travel delays.

Police had found and defused the other devices before they ignited.

Authorities believe the arson attacks are the work of leftist extremists, after a group identifying itself as the Hekla Reception Committee Initiative for more Eruptions in Society claimed responsibility for Monday's attack. In an online posting, the group said it is protesting Germany's role in the war in Afghanistan.