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If you are applying for EAD for a pending !-485 app., you need to first apply to VSC. If 90 days have passed since the RD of the EAD app, you can walk in and fill out a duplicate I-765 (no fees) and show the receipt of the I765 app from VSC. They then send you a letter to come in for a photo and issuance of the card. The process at Baltimore takes ~2 weeks.
If you are applying for EAD in another category, such as J-2 or asylee I am not sure as some local INS accepts and other refer to the service center which is VSC if you are living in MD.