Fang Zheng did not need to think to know who had sent this letter.
The reason was simple: he had planted an eye on Vermouth.
He could even watch Vermouth while she bathed!
Fang Zheng even knew Vermouth's plan—she wanted to use this letter to lure Fang Zheng away from the mountain. She had sent the same letters to Kogoro Mouri and Edogawa Conan, and in Vermouth's eyes, since Fang Zheng was a senior consultant for the Metropolitan Police Department, he certainly would not stand idly by when faced with something like this...
Well, Fang Zheng actually would, especially considering that if Conan and Kogoro Mouri really went, he could just find a movie theater and watch a film.
Of course, if Fang Zheng did not go, Vermouth had another move: go straight to the Metropolitan Police Department and destroy the Skynet server. As Skynet's chief designer and maintainer, Fang Zheng would certainly have to show up. Once he was separated from the Miyano sisters, Vermouth would swiftly kill Miyano Akemi, then use Akemi's voice to lure Haibara Ai out and kill her before quickly making her escape.
It had to be said that the entire plan was quite good, and with Vermouth's skills, it would indeed be easy for her to succeed.
The letter to Conan was also meant to lure him away. Haibara usually spent her days with Conan and the Junior Detective League, so to be safe, it was best to lure Conan away as well.
Therefore, to be safe, none of the three Invitation Letters had signatures, and the letter sent to Conan was addressed to Kudo Shinichi.
That made them feel like the vague pleas for help or warning letters Conan and Kogoro Mouri had received before. Given their personalities, they would definitely rush over