On Friday, a Turkish judge ordered the rearrest of four of those 10, who had been released awaiting trial. Critics say the Turkish judiciary is no longer independent from Mr. Erdogan’s government, after a vast crackdown on opponents that purged around 150,000 public employees, including more than 4,000 judges and prosecutors.
This week, the German foreign minister, Sigmar Gabriel, called Mr. Steudtner’s arrest “absurd” and said it showed “that German citizens are no longer safe from arbitrary arrests” in Turkey.
In remarks published in Bild on Friday, the finance minister, Wolfgang Schäuble, said that if Turkey “doesn’t stop these games, we will have to tell people: ‘You are traveling to Turkey at your own risk, we can no longer provide guarantees.’ ”
Also on Friday, two Germany news channels said that they would no longer run ads that feature the soccer star Lukas Podolski encouraging investors: “Come to Turkey. Discover your own story.”
The stations, N-TV and N24, told the German news agency DPA they had decided to cancel the ads after Mr. Gabriel’s statement on Thursday questioning Turkey’s safety.
The Germany government is also furious about the detainment of nine other German citizens in separate cases, including two journalists, Deniz Yucel and Mesale Tolu. Turkish politicians have also provoked their German counterparts by accusing them of “Nazi practices” and by refusing to allow German parliamentary delegations to visit German soldiers carrying out operations against the Islamic State from two Turkish military bases.
For his part, Mr. Erdogan is angry that Germany has granted asylum to former Turkish Army officers and other officials accused of playing a role in last year’s coup attempt in Turkey. Mr. Erdogan also says that Germany harbors members of the Kurdistan Workers’ Party, or P.K.K., which has waged an insurgency in southeastern Turkey for several decades.
“The government who is hiding Turkish terrorists in Germany should first explain this,” Mr. Erdogan said on Friday. “Why are they hiding in Germany? How they can explain the material support given to them?”
Another irritant for Mr. Erdogan is the opening of investigations by German prosecutors into German-based representatives of Turkey’s religious affairs directorate. The representatives are accused of spying on Turks living in Germany, home to around three million people of Turkish origin.