tomato seedlings dying after transplant to pots

This is my first year of trying to plants seeds and grow them into tomato plants. I had a number of seedlings that had 2 true leaves on them and were about 4-6" tall that I transferred to pots a week or so ago. They've been under the lights since it's still too cold here to put them outside, and I noticed that yesterday evening there were about 5 of them that were folded over - almost like someone crushed the stem where it met the soil. There were all standing on their own after I transplanted them and I'm confused as to what may have happened.

Any thoughts/ideas/suggestions are greatly appreciated. Thanks in advance.

Sounds like "damping off" to me - here's a link:

http://tomclothier.hort.net/page13.html

In past years, I've started seeds indoors, but this year I'm growing all seeds from scratch straight in the garden because I really don't have the time to take care of them under lights.

To prevent damping-off try this trick:

Plant your seeds as usual - then put a very thin layer of vermiculite over the soil. For some reason this seems to prevent damping-off - it's not foolproof, but I've had much greater success by doing this over the years.

Would perlite work the same as vermiculite?

Yep, sounds like dampening off. This this:
1. Make sure your seed is from a reputable source or grow your own.
2. Make sure the container that houses the seeds/plants is clean. Ex: Mix Clorox and water to clean the containers before use.
3. Don't over-water and water from underneath to prevent moisture from getting on the leaves. Over-watering can be destructive.
4. When you transplant them (when they are tender and young), be sure to leave the roots intact and try not to disturb them.
5. Don't keep the temperature around the seeds/plants too high. My temperature varies from 55-65 degrees while they are in the house and under the lights most of the time.
If these suggestions don't work, I would try using the disease preventing methods. Hope this helps.

Damping off. Caused by the young plants sitting in soil that is too wet.

These are all great tips.
Besides making sure that my seed-starting containers are clean (water and bleach) and that I water from beneath, I also set up a few small fans to blow gently over the new seedlings. They are the type that can clip to the sides of my shelving. I just purchase them from our local drug store. Not sure what the science is--maybe the soft flow of air makes the seedlings stronger? Maybe it keeps the temps around 55-65 F?
Anyway, I've not had damping-off so far.

I think the big benefit of air flow is that moving in the breeze thickens up the stems...

So the fan should be pointed over top of them, not directly at them? I want to make sure that I understand this correctly.

Yes, the air flow helps to thicken the stems. I use fans through most of the process, even when they are ready to transplant. It helps to circulate air around them too, obviously, which helps to keep diseases that love excess moisture at bay. My fans push air directly into the plants, not just over the tops of them. I've had great success (knock on wood) by following the steps listed above with the addition of air circulation.

cdouglass - I don't know if perlite would work as well as vermiculite - I've never tried swapping the two. If you try perlite, let us know how it works out.