what is beheading my tomato seedlings each night?

I have about 270 tomato and tomatillo plants. Over the last week I have found 1-3 of my seedlings completely beheaded. At first I thought: mice, then possibly rabbits, but now possibly cutworm? I am just curious to find out if this has happened to others and what you have done about it? Thanks! Help me troubleshoot!

I would start by getting out there with a flashlight at night and having a look. You might well find some cutworms in action. You can often catch them during the day, curled up asleep just under the soil surface near the plants they have munched the night before. Since tomatoes plants are, at least to some extent, poisonous, it would seem that rabbits or mice would pass them by in favor of other things. Insects and mammals vary greatly as to what is edible and not to them......

Yes, it was cutworms! Two nights ago we went out with headlamps and found a couple of them. So we surrounded the tomatoes with diatomaceous earth and have not had any more deaths in the last 2 nights. Horray!