A North Korean spy who came to South Korea last year, posing as a defector, has been caught and indicted. The agent is the third North Korean woman to disguise herself as a defector to infiltrate the South.

South Korea’s counterespionage officers will have to keep their guards up against North Korea’s attempt to dispatch spies as defectors. The job will be difficult, given that more than 2,000 North Koreans defect to the South each year. It is not rare for some of them to slip out of the country and return to the North after a few years here.

The mission for the agent of the North Korean Security Ministry this time was not limited to gathering intelligence. In 2001-07, prosecutors said, she put counterfeit $100 U.S. bills, totaling $570,000, into circulation in China. They said the bills were printed in North Korea.

While in China, they said, she provided a homestay for South Koreans as a means of collecting intelligence. They said she also posed as a relative to a Korean-American presumed to be a U.S. intelligence officer and spied on him when he came to see her.

When she came to the South via Thailand last year, she lied to immigration officers that she decided to defect when a man from the South who she lived with returned home. After months of surveillance and investigation, the National Intelligence Service arrested her on charges of espionage in May this year.

The South Korean agency did not say what intelligence the spy passed to the North. But in 2008, a woman agent was caught obtaining confidential military information from Army officers. Another woman agent was arrested on charges of taking sensitive materials from a Seoul Metro official in 2010.

Of course, espionage is not limited to female agents. The National Intelligence Service recently arrested a businessman who served jail term for his pro-Pyongyang activities and refused to renounce his allegiance to North Korea, on charges of obtaining classified information on advance military technology for the North. It also arrested a Korean-New Zealander on similar charges.

No less worrisome than espionage by North Korean agents are pro-Pyongyang South Korean lawmakers affiliated with the United Progressive Party. Sensitive national security information that they have access to must be prevented from finding its way to the North Korean communists.