i checked amcas and couldn't find the answer....

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You submit letters after receiving secondaries. If a school you're applying to uses AMCAS' letter system you must submit the letters to AMCAS and then release them to schools.
 
You can send letters to AMCAS as soon as it opens I believe. However, schools that use these service (at least this cycle) were few and far between. A good rule of thumb would be to have your letters ready to be sent (i.e. through Interfolio) by the time you submit your primary AMCAS - some schools will get back to you very fast (like GW) and you shouldn't have to wait to fill out their application due to a delay in your letters!
 
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