In the letter, Ferman says her office started an investigation after being approached by a recently fired Washington staffer, who detailed political work done for Washington by Senate employees on state time.
"As this investigation has progressed, we determined that most of the activity occurred outside of Montgomery County," Ferman wrote to Kane. "As such, I believe your office is in the best position to continue to investigate these allegations against Sen. Washington and, if necessary, to prosecute the offenses."
Carolyn Myers, a spokeswoman for Kane, said her office is reviewing the motion "and will file a timely response."
Washington is accused of using her Senate staff to plan birthday parties for her at the Belmont Mansion in Philadelphia.
The events doubled as her annual political-campaign fundraiser.
Hockeimer's motion notes that Washington's campaign committee is based in Philadelphia, as are the senator's staffers, former staffers and fundraiser party vendors expected to be called as witnesses in the trial.
That includes Sean McCray, Washington's former chief of staff, who first went to the Montco D.A. with his concerns and later cooperated with the grand jury that investigated the case.
"When issues concerning Mr. McCray's behavior arose following his termination from Sen. Washington's offices, the Senate contacted the Philadelphia Police Department, not the Montgomery County Police Department, to address the issues," Hockeimer wrote in yesterday's motion.
McCray, who now lives in Ohio, is suing Washington in Montgomery County, claiming she wrongfully fired and defamed him while violating the state's whistle-blower law.
On May 20, Washington lost a three-way Democratic primary election for the 4th Senate District seat she has held since 2005.
The 4th District stretches from Logan in Philadelphia north to Willow Grove in Montgomery County.
On Twitter: @ChrisBrennanDN