More than 18 months after a Wichita, Kan., abortion doctor was gunned down in his church, a federal investigation into a possible conspiracy continues in Kansas City, Mo.

Federal agents have questioned more people in the past few weeks, while a grand jury convened after the murder of George Tiller is still under way.

The focus, according to those who have been interviewed, still appears to be on a Bible-study group that Tiller's killer attended.

Tiller was shot to death in May 2009 in the foyer of his Wichita church while serving as an usher. Scott Roeder, of Kansas City, was convicted of first-degree murder and sentenced to life in prison with no chance of parole for 50 years.

In October, two of Roeder's former roommates, who were members of the Bible-study group, said that they and several other members had testified before the grand jury in late September.

The questions they were asked, they said, focused on whether Roeder had acted alone.

More of Roeder's former associates say they have been contacted by authorities since then, including two additional members of the study group, which met in members' homes on Saturdays. Those attending described themselves as Messianic Jews who, unlike mainstream Jews, believe that Jesus was the Messiah.

Abortion-rights advocates have been pressing the Justice Department to investigate the possible existence of a network of anti-abortion extremists involved in clinic-related violence, including the murder of Tiller.