The use of the apartment is now part of a broader criminal investigation of Dougherty, who is the leading candidate for the State Senate seat being vacated by Vincent J. Fumo.
Federal agents are trying to determine if Dougherty was truthful on the disclosure forms about the length of his stay at the Dockside, sources said.
DePaul, who recently testified before a grand jury, isn't suspected of any criminal wrongdoing.
The Blue Bell developer is now an investor in the proposed Foxwoods casino, planned for a riverfront site in Dougherty's Pennsport neighborhood. DePaul joined the casino venture after Dougherty's stay.
The casino, opposed by many in the neighborhood, has become a major issue in the First District campaign.
DePaul's lawyer, Mark Sheppard, said his client has cooperated with authorities but he declined to elaborate.
"This will advise you that Mr. DePaul cannot affirm any matters occurring before a federal grand jury, as such matters are confidential," said a letter from Sheppard and attorney Richard A. Sprague.
Dougherty's lawyer, Henry Hockeimer, and his campaign spokesman, Frank Keel, declined to comment. Neither Dougherty nor DePaul responded to requests for interviews.
The grand jury, court records show, is investigating whether Dougherty violated tax laws and a labor law that prohibits union leaders receiving gifts from contractors who employ union workers.
(The labor law apparently would not apply to the apartment matter because DePaul's development company does not directly employ electrical union employees, according to people familiar with the issue.)
Last year, the grand jury indicted Dougherty's childhood friend, electrical contractor Donald "Gus" Dougherty Jr., on 100 felony counts, including two allegedly involving John Dougherty's properties.
