The prosecution on Wednesday indicted Kim Ki-jong, a leftist extremist who attacked US Ambassador to South Korea Mark Lippert last month, on charges of attempted murder, assault on a foreign diplomat and business obstruction.

The authorities did not file any charges of violating the National Security Law, citing a lack of evidence. But they said their probe would continue to verify the allegations that Kim had violated the law by possessing various publications carrying "enemy-benefiting" contents.

The 55-year-old assailant attacked Lippert with a paring knife during a forum in downtown Seoul on March 5. At the scene, Kim said that he carried out the attack to stop the South Korea-US military drills that were strongly opposed by North Korea.

The attack left an 11-cm wound on Lippert’s face and another injury on his left arm. The ambassador immediately received surgery and is currently recovering.

The prosecution filed an attempted murder charge as the depth of the wounds and the paring knife indicated that Kim had premediated the attack with an intention of killing the top US envoy. But Kim has claimed that he did not mean to murder Lippert.

The investigators questioned two surgeons who conducted the surgery on Lippert to analyse the wounds and prove that Kim attempted to murder him with the knife.

As for the possibility of complicity, the prosecution said Kim appears to have carried out the attack alone.

Meanwhile, on Wednesday, Lippert wrote his first blog post since he left the hospital on March 10. He said that the attack has further strengthened the long-standing alliance between Seoul and Washington.

"This incident has only strengthened our belief in the unbreakable bond that exists between the United States and the Republic of Korea," he wrote.