China-based hackers breached the office computers of Democratic Sen. Bill Nelson of Florida, the senator’s office confirmed March 20.

In three separate attacks, two in March and one in February, cyberhackers targeted the work stations of Nelson’s foreign policy aide, his deputy legislative director, and “a former Nelson NASA adviser,” Nelson’s office said in a statement.

The hackers did not steal any classified information, which is not stored on office computers, the statement said.

A Nelson aide said the attacks were traced to China through Internet Protocol (IP) information, which could have been masked. The Office of Senate Security and the Senate Sergeant at Arms Information Technology Security Branch responded to the attacks, the aide said, by wiping clean malicious code from the affected systems.

Nelson first disclosed the attacks March 19 at an Armed Services Committee hearing that featured testimony by senior military officials with domain over cyberwarfare.

“I have had my office computers invaded three times in the last month, and one of them we think is very serious,” Nelson said at the hearing.

At another hearing the same day, this time held by the Commerce, Science and Transportation Committee, Nelson said his computer seemed to be “talking to a computer in some international arena.”

China has been blamed by lawmakers for several cyber-attacks in recent years, including intrusions into the offices of Rep. Frank R. Wolf , R-Va., in 2006.

Nelson’s office also referenced a major cyber-intrusion in August that infected the communications networks being used by the presidential campaigns of then-Sen. Barack Obama , D-Ill. (2005-08), and Sen. John McCain , R-Ariz. A senior campaign official said there was significant evidence that Chinese hackers were responsible for the intrusions.

Hackers working from China have been suspects in several high-profile cyber intrusions, including a November 2006 attack that shut down computer systems at the Naval War College in Rhode Island for several weeks.

That same year, Chinese hackers infected the systems at the Commerce Department’s Bureau of Industry and Security, which controls technology export rules, forcing the computers to be replaced altogether.

Origins Tough to Trace

Experts say it is extremely difficult to determine whether such attacks originate from government entities or independent agents. But some officials have admitted that they suspect the Chinese government is behind the attacks, due to the level of sophistication and the nature of the information targeted.