[sunshineband]

If it were rabbits I would have thought they would have attacked other things nearby like lettuce.  Nothing just the carrot seedlings.

Mike.

Although yes, rabbits would have munched other things, they actually don't usually like lettuce

[Salmo]

In my experience the most likely culprits are slugs, both surface living ones and underground keel slugs.

Things got so bad one year I sowed slug bait pellets with the carrot seed to kill off those little black slug under the ground 😲

I have had to do the very same with my second sowing this year.  The first all germinated lovely then they vanished as if the bed was empty.  I cursed that I had neglected to put some slug pellets on,  then re-sowed with pellets alongside.  I don't like using slug pellets, but use the organically allowed ones and as the bed is netted against carrot root fly anyway, so birds and animals can't get at them.  Result is the second sowing is now growing well.  Only 1 visible dead slug, I think the main culprits are the underground keel slugs.

The organic approved slug pellets contain ferric phosphate which is a poison only to slugs and snails. Other creatures are unharmed if they eat it and it passes straight through.  For that reason they do not need the netting precautions needed with ordinary pellets which contain metaldehyde, a thoroughly nasty poison. At last my local garden centre has started to put the organically approved ones at the front of the shelf.