Ten retired admirals were arrested in Turkey on Monday, the day after the publication of an open letter criticizing President Recep Tayyip Erdogan’s plans to dig a canal in Istanbul.

About a hundred ex-naval officers signed the open letter. They say the canal threatens the free passage of civilian ships in the Bosphorus, the strait between the Sea of Marmara and the Black Sea.

They fear that Turkey wants to withdraw from the Treaty of Montreux, which has regulated free passage in the Bosphorus and the Dardanelles since 1936. After all, the new canal could serve as an alternative Bosphorus that is not covered by the Montreux treaty.

Immediately after the publication of the open letter, there were strong reactions from the government. “Not only those who signed the letter, but anyone who encourages them will have to answer in court,” Erdogan’s communications director Fahrettin Altun wrote on the microblogging site Twitter.

And the attorney announced an investigation into “conspiracy against the security of the state and the constitutional order.”

Turkey announced plans in mid-January to start construction of a 45-kilometre channel between the Black Sea and the Marmara Sea this year. The waterway should relieve the Bosphorus, the natural connection between the two seas.