What happens is, the alien commits a crime and is arrested. Our (the INS) Criminal Investigators or Immigration Agents are supposed to visit the local jails and see if their are any illegals. If they find one, they serve a detainer to the local authorities, that requires them to contact the INS when the alien is to be released. At that time the INS goes and picks up the alien and takes the alien into INS custody for the INS violation.
I would bet that this is what happened in these cases.