Attorney General Tom Reilly is seeking a special grand jury to grill more than a dozen key witnesses in the Big Dig manslaughter probe, moving swiftly after documents revealed widespread knowledge of ceiling problems in the tunnel where Milena Del Valle was killed, a law enforcment source said. The grand jury, expected to be convened Oct. 3, will hear evidence for six months as investigators seek criminal indictments against officials involved in construction of the I-90 Seaport Connector Tunnel. Several key officials have already retained lawyers, the source said. “Nothing we have seen to date takes us off the path...