By Billy Stockwell, Sebastian Shukla, Chris Stern and Matthies Otto, CNN

(CNN) — Eighteen people were injured, four critically, in a knife attack Friday in the German city of Hamburg, authorities said.

Hamburg police said they have arrested a 39-year-old German woman, who they believe acted alone in carrying out the attack at the city’s Central Station, after a major police operation. She is in police custody and expected to be brought to a court on Saturday, police said.

Seven people were “badly injured,” while seven others were “lightly injured,” police said. The victims are aged between 19 and 85 years old.

The four critically injured, who include a 24-year-old female, 24-year-old male, 52-year-old female and 85-year-old female, are in a stabilized condition, police said in an update on Saturday.

Investigations into the incident are ongoing. Police said there was “concrete” evidence that the suspect suffers from a mental illness.

In a statement late on Friday, Hamburg police said the attacker had acted “apparently indiscriminately, stabbed passersby, injuring some of them critically.”

Video footage from the scene filmed by CNN’s affiliate RTL Germany shows people in forensic suits inspecting the area while police gather in the train station.

Following the attack, Germany’s Friedrich Merz thanked the city’s emergency responders for their assistance and said that his “thoughts are with the victims and their families,” according to federal government spokesperson Stefan Kornelius.

Federal Minister of the Interior Alexander Dobrindt also said his thoughts were with the victims and thanked the emergency services. “It is shocking when travelers are attacked in a treacherous and cowardly manner,” Dobrindt said.

Hamburg’s Central Station is the busiest passenger railway station in Germany, with more than 550,000 travelers per day, according to the city’s website.

Germany has faced rising rates of knife crime in recent years, with some incidents stirring up tensions over migration ahead of parliamentary elections in February.

Some in the country were concerned by a series of high-profile attacks allegedly carried out by asylum seekers or migrants.

In January, a 28-year-old man from Afghanistan was arrested following a knife attack in the German city of Aschaffenburg in which two people were killed, including a toddler.

Last year, a Syrian man turned himself in and confessed to stabbing to death three people and wounding several others at a festival in the western German city of Solingen.

This story has been updated with additional information.

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CNN’s Frederik Pleitgen contributed to this story