They put him in custody, because there was no clear instruction about what should be done to him.

You must place them ¹  in the tent of meeting before the ark of the covenant ²  where I meet with you.

Then Moses placed the staffs before the Lord in the tent of the testimony. ¹ 

For if you turn away from following him, he will once again abandon ¹  them in the wilderness, and you will be the reason for their destruction.” ² 

“‘Then a man who is ceremonially clean must gather up the ashes of the red heifer and put them in a ceremonially clean place outside the camp. They must be kept ¹  for the community of the Israelites for use in the water of purification ²  – it is a purification for sin. ³