D.C. Code § 23-1504: Exemption from arrest.

(a)

Any person who comes into the District of Columbia in obedience to a summons directing him to attend and testify in the District of Columbia shall not, while in the District of Columbia, pursuant to the summons, be subject to arrest or the service of process, civil or criminal, in connection with any matter which arose before his entrance into the District of Columbia under the summons.

(b)

Any person who is in the process of passing through the District of Columbia for the purpose of proceeding to or returning from a State which has summoned him to attend and testify shall not be subject to arrest or the service of process, civil or criminal, in connection with any matter which arose at some other time.