Emergency! Chick lethargic, laying on side, not moving

[MrsOks]

HELP! We had a chick (about 3 months old) pass away last Friday. She had been really lethargic, probably her whole life, just hanging out in the corner. She got really bad and passed away.
This morning, the three others were happy, jumping all around, flying and trying to get out of their house.
Tonight, we can home and another of our girls (same breed and hatch- if that matters) was showing the same signs.
Laying slightly on her side, legs sort of out stretched, lethargic, and letting the other walk all over her. (literally)
When I pick her up, she doesn't seem to care or move much, which is very unlike her. She usually protests touching of any kind. I separated her from the others, just in case. I tried to stand her up in her new "quarters". She stood for a bit and then kind of kicked her legs and flopped to her side. She CANNOT grip my finger if placed under her foot. Her head is now down and she's not moving at all.
We gave her an antibiotic (oxytetracycline powder), put her under a light, and are letting her rest.
I have no idea what it could be or what else I can do? ANY advice would be great!
Thank you in advance!

 

[Wyorp Rock]

Welcome to BYC.
Was she vaccinated for Marek's?
Here's some info on Marek's and some vitamin deficiencies/illnesses that can mimic it.

https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq

 

[MrsOks]

She was not vaccinated for Marek's. Unfortunately, she passed away last night. We are sending her for a necropsy to find out WHAT happened. After losing two chicks in a week, I'm worried for my remaining two!

Thank you for the info on Marek's though. It was helpful.

 

[Wyorp Rock]

Sorry for your loss.

A necropsy is a good idea.