PHILADELPHIA — Although she built a 139-count corruption case against a powerful state senator, an FBI agent testified Thursday that she could never get at the crux of the case — whether he extorted corporate donations — because of missing e-mails.

The FBI recovered more than 83,000 e-mails from computers and Blackberries in searches of Sen. Vincent Fumo's two state offices and those of a charity run by his former aides.

They found e-mails suggesting Fumo was using staff for campaign and personal work, and others concerning his alleged fraud on the charity, she said.

But they found no correspondence related to his negotiations with Peco Energy and Verizon Pennsylvania over deregulation issues.

"There was nothing," said Special Agent Vicki Humphreys, the last witness in the government's three-month case.

Other witnesses have testified that Peco gave $17 million to the Fumo-linked charity, Citizens Alliance for Better Neighborhoods, as it negotiated with Fumo over energy deregulation.

And a Verizon official testified that Fumo secretly sought $50 million in donations and favors, including millions in business for his family bank and law firm.

Verizon refused the vast majority of the requests. Fumo, meanwhile, was never charged with extorting either company.

Instead, he was charged in February 2007 with defrauding the state senate, the charity and a seaport museum of more than $3.5 million, and with obstruction for allegedly destroying e-mail evidence after the FBI probe was under way.

Fumo has pleaded not guilty, and maintains that he continued his e-mail deletion policy amid the probe only after consulting his then-lawyers.

The defense is expected to begin its case next week.

Humphreys told jurors Thursday that co-defendant Lenny Luchko — the fiercely loyal computer technician in Fumo's district office in South Philadelphia — was trying to "wipe" or destroy data from his laptop even as the FBI searched the office on Feb. 18, 2005.