SEOUL, South Korea — A hacking specialist at South Korea’s National Intelligence Service who was found dead left a suicide note denying that his team spied on South Korean citizens’ cellphone or other online communications, the police said on Sunday.

The 45-year-old agent, who was identified only by his last name, Lim, was found dead in his car near Seoul on Saturday in what appeared to have been a suicide. His death came as the political opposition demanded an investigation into suspicions that the spy agency had been intercepting the cellphone and computer communications of South Korean citizens, including government critics, using software it purchased from the Italian company Hacking Team.

Hacking Team was itself hacked, with large amounts of emails and other internal company data leaked online early this month. The data showed that one of the company’s clients was “South Korean Army Unit 5163,” which is widely believed here to be one of the cover addresses for the National Intelligence Service.

Last week, the agency admitted buying hacking programs from the Italian company in 2012. But it said the purchase was for research purposes as it tried to bolster its spying on North Korea. South Korean bloggers, news outlets and opposition parties have cited Hacking Team’s leaked data to suggest that the agency may have spied on the country’s own citizens.