A 33-year-old man has been arrested for allegedly threatening to kill Mark Lippert, the U.S. ambassador in Seoul, South Korean police said Friday. The allegations come four months after Lippert was attacked with a knife in Seoul.

The man, who has been identified only by surname Lee, is suspected of posting a message on July 8 on the White House’s website threatening to “kill” 42-year-old Lippert with “nuclear poisoning,” the Seoul Metropolitan Police Agency said in a statement, according to the Associated Press (AP).

Police arrested Lee from his Seoul house on July 14 after they received an investigation request from the White House, Yonhap News Agency reported.

The man has denied the allegation, but the police reportedly maintain that they recovered a draft of the threat written in English and its screenshot on his laptop. Authorities did not say Lee’s reason behind posting the threat that had too many grammatical errors, the AP reported.

According to the police, Lee is reportedly unemployed and lived in seclusion spending most of his time surfing the Internet. Lee could be jailed for five years if found guilty.

In March, 55-year-old Kim Ki-jong was charged with attempted murder of Lippert after he attacked the ambassador with a knife at a breakfast function in Seoul. Lippert sustained deep wounds on his face and arm, and received 80 stitches. Kim told investigators that he attacked Lippert to protest the South Korea-U.S. army drills, which damaged the relationship between the two Koreas.