BERLIN-- Police on Thursday found bottles filled with a potentially explosive mix of liquid and powder beside train tracks in southwestern Berlin on Thursday, the 16th firebomb discovered in four days.

Investigators found the bomb, which hadn't exploded, hidden in the grass next to the tracks. It was made of two bottles with yellow and brown liquids and two blue bags filled with powder.

Experts were on hand to defuse the device, federal police spokesman Meik Gauer said.

Sixteen firebombs have been found in nine locations around the capital since Monday, forcing hundreds of trains to be delayed and frustrating thousands of passengers through partial shutdowns.

One went off, damaging a track west of the city, and another ignited but did not explode.

Nobody has been injured.

It was not clear when the most recent bomb was placed; authorities said that all of the devices could have been planted at the same time over the weekend.

A previously unknown leftist group has claimed responsibility for one of the firebombs.