Vincent Gray and Lorraine Green at his mayoral campaign announcement on April 24. (Marvin Joseph /The Washington Post)

Here’s a thought: It might be because Issa knows one of the characters in this saga fairly well — Lorraine Green, who chaired Gray’s campaign and transition committees and also works as Amtrak’s top HR executive. Brown has accused Green of orchestrating a deal to trade a city job and cash payments for campaign-trail attacks on Mayor Adrian M. Fenty (D).

When Issa was ranking member of the House Oversight Committee, he and Sen. Charles Grassley (R-Iowa) investigated the 2009 forced retirement of Amtrak’s inspector general, Fred E. Weiderhold Jr. After Weiderhold was ousted by Amtrak’s board — in violation of federal law, the Republicans found — he was replaced on an interim basis with none other than Green.

Sound like a problem to task an Amtrak executive with probing the rail carrier for management lapses? You bet it does: Green’s “inherent conflicts of interest, lack of independence and lack of IG experience rendered her an inappropriate choice for the position,” Issa and Grassley’s report found.

Green, in fact, told investigators that she delayed the release of a report that found “weaknesses in the system used by the Amtrak HR Department to ensure that Amtrak employees completed mandatory training in a timely manner.” That was squarely within Green’s HR realm.

The Washington Times covered the report in December, as Green managed Gray’s transition. Green was spared from the Republicans’ harshest criticism — Amtrak board members got the worst of it, and they never called for Green’s ouster, only that she give up the IG post — but the probe certainly put her on the congressional radar.

Green, 65, announced her retirement from Amtrak earlier this month, days before Brown made his allegations public.

Brown said he received his first cash payment from Green at Union Station, where Green worked. The Washington Post has not been able to independently verify any payments, and Green has vigorously denied Brown’s charges.