Some of Mayor Bill de Blasio's closest colleagues and allies have been subpoenaed in a political fundraising probe.

Investigators are looking into money funneled to Democrats running for the state Senate.

"As I've said many times, we hold ourselves to the highest standard of integrity," de Blasio said following news of the subpoenas. "Everything we've done from the beginning is legal and appropriate." 

John Kaehny, an advocate for increased transparency in Albany, says the mayor is being investigated over a common practice used by other politicians.

To get around campaign finance laws that limit how much an individual can contribute to a candidate, donors can give money to political committees instead.

"This is about using a very heavily used and well-known loophole in campaign finance laws," Kaehny says. "It's a backdoor way to funnel very large amounts of money from big donors to candidates."

Some Republicans in the state Senate, like Sen. Terrence Murphy, who represents an area in the Hudson Valley, have accused de Blasio of funneling the money to Democratic challengers seeking to oust GOP incumbents.