Authorities in Berlin have raided the homes of suspected members of Islamic State (IS, formerly ISIS), seizing evidence and data storage devices, the city’s public prosecutor's office has said. It remains unclear whether any arrests were made.

Some 130 police officers were involved in the operation, searching nine addresses in Berlin and Saxony-Anhalt, security officials said in a statement, though they did not specify if any arrests had been made. Local media, however, reported several people detained.

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“The General Prosecutor's Office in Berlin is investigating four suspects, aged between 18 and 21, who are suspected of membership in a terrorist organization (IS) as well as of preparing a serious crime against the state,” the statement reads.

Two are suspected of travelling from Berlin to an IS-controlled zone - through Istanbul - in November 2016, while another is suspected of receiving combat and explosives training from IS terrorists. The fourth is believed to have helped the others travel there, by driving them to an airport, the police said.

A number of data storage devices were seized during the raid, as well as some other evidence, according to the statement. A suspicious bag was searched during the operation, Berliner Zeitung reported. The bag, however, contained only a bottle of wine.

Bild reported that the suspects could be linked to Anis Amri, the man who plowed a truck into a Christmas market in the German capital last year, killing 12 people and injuring dozens more. Earlier this month, the families of the victims wrote an open letter to Angela Merkel, accusing the chancellor of "inaction" in the face of terrorism.

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The Thursday raids come just four days after a "large amount" of ammunition was discovered near a Berlin Christmas market on Sunday, prompting police to sweep the area. Around 200 rounds of new 7.65-caliber ammunition were reportedly found in the stash. The cartridges were in sealed plastic bags and in their original boxes.

Last month, six Syrian asylum seekers who were suspected of being IS members were detained for allegedly plotting an attack against a "public target." Local media reported that the suspects had targeted a Christmas market.

On Sunday, German Intelligence chief Hans-Georg Maassen said the country's security services are facing a record number of Islamists. The number jumped from 9,700 to 10,800 over the past year, he said. Germany has been on high alert since last year's Christmas market attack.