וַיִּכְתֹּ֣ב אֲלֵיהֶם֩ סֵ֨פֶר ׀ שֵׁנִ֜ית לֵאמֹ֗ר אִם־לִ֨י אַתֶּ֜ם וּלְקֹלִ֣י ׀ אַתֶּ֣ם שֹׁמְעִ֗ים קְחוּ֙ אֶת־רָאשֵׁי֙ אַנְשֵׁ֣י בְנֵֽי־אֲדֹנֵיכֶ֔ם וּבֹ֧אוּ אֵלַ֛י כָּעֵ֥ת מָחָ֖ר יִזְרְעֶ֑אלָה וּבְנֵ֤י הַמֶּ֙לֶךְ֙ שִׁבְעִ֣ים אִ֔ישׁ אֶת־גְּדֹלֵ֥י הָעִ֖יר מְגַדְּלִ֥ים אוֹתָֽם׃
耶戶又寫第二封信給他們，說：「你們若歸順我，聽從我的話，就要把你們主人眾子的首級取來，明天這個時候到耶斯列來見我。王的眾子共七十人，都在城中撫養他們的權貴那裡。」
Rashi
The sons of the king [number] seventy men, and the great men of the city, etc. All this was written in the letter, “And I know their count, seventy men, and they are with the elders of the city who are raising them.2Alternatively, “now the kings sons, etc.” until the end of the verse was not written in the letter. Rather, it is a comment by [Yirmiyahu] the author of the Book of Kings.—Radak