A Thursday filing by federal prosecutors reveals an interesting strand of research by Jared Lee Loughner's defense attorneys.

In 22 subpoenas, defense attorneys requested birth and/or death certificates of people who appear to be Loughner's family members or ancestors. The subpoenas were issued July 8 to the state of Illinois' division of vital statistics.

Why? The records don't explain the reason, and Loughner attorney Reuben Camper Cahn, who requested the records, has not returned a call. But attorneys I've spoken with can only think of one explanation: Loughner's attorneys want to trace mental illness in his family.

Evidence of a family history of mental illness could be useful in an insanity defense or in arguing for "mitigation" -- that is, a lesser sentence than the maximum, which in this case could be death.

Among those whose birth certificates the defense is seeking is Judy Wackt, whom the Washington Post identified as a first cousin of Loughner's mother, Amy. The Post quoted Wackt as saying there is a family history of mental illness on Loughner's mother's side.

There's a history in the family of what they used to call manic depression, which I guess they now call bipolar disorder. My mother battled depression. One of her sisters had extreme bouts. She'd be okay, then she'd dissolve over time. Wouldn't leave the house. Wouldn't bathe. Wouldn't interact with her husband or children.

Others whose vital records the defense sought are Loughner's maternal grandparents, Laurence Totman and Lois Bleifuss. The earliest record that defense attorneys sought in this set of subpoenas was an 1893 birth certificate belonging to a woman who apparently was Loughner's great grandmother, Lois Johnson Totman.

Online ancestry experts have looked into Loughner's family tree largely because of a friend's comment to a reporter that Loughner's mother is Jewish. As a result, analyses such as this have been produced by a variety of online researchers, seeking to verify or disprove the comment.

Prosecutors included the subpoenas as part of a motion, attached, to force defense attorneys to follow subpoena rules set out by the court. The other subpoena was for a lease agreement at the Tierra Rica Apartments, 3201 W. Ina Road.