And they placed him in custody, for it had not been specified what should be done to him.
Torah Temimah / Sifrei / Sanhedrin 78b: they knew he was liable to death, not which death.
English: “For it had not been specified what should be done to him” — by which death he should die; but they knew that one who desecrates Shabbos is subject to death.
Gap between known liability and unspecified form of death.
Custody waited for the specific method; the principle of death for Shabbos desecration was already clear.