Federal authorities investigating the fund-raising operation of the New York City comptroller, John C. Liu, had their eyes on him as early as 2009 and obtained judicial approval to listen in on his phone conversations for a year and a half, according to new court filings.

The filings, made Monday by lawyers for two defendants charged in the case, say that Mr. Liu and a top aide, Mei-Hua Ru, were targets of electronic surveillance by the government in 2010 and 2011.

The existence of that investigation has been known for some time, but the filings by lawyers for the defendants, a former Liu fund-raiser, Xing Wu Pan, and Mr. Liu’s former campaign treasurer, Jia Hou, offer new details about the inquiry.

The authorities obtained 10 wiretaps on six different phones belonging to Mr. Liu, Ms. Ru, Mr. Pan and two others, one filing says, adding that “the government intercepted thousands of calls on the wiretaps.” The filing makes clear that the government began the wiretaps in March 2010.