A man who claimed he was instructed to throw a hammer at Prime Minister Lee Hsien Loong and threatened to "stage an attack" on him in four Facebook messages was yesterday jailed for 15 months.

Tan Yeong Hong, 33, had pleaded guilty to a charge of criminal intimidation and to two counts of hurting two policemen.

At the time of the offence, he was suffering from paranoid schizophrenia, which had been undiagnosed and untreated for 16 years. But a report from the Institute of Mental Health found that he was not of unsound mind and was fit to enter a plea in court.

In passing sentence, Community Court Judge Mathew Joseph noted that psychiatric reports found Tan to be a "high risk" to all around him, including his parents and strangers.

"It is quite clear that you require psychiatric follow-up and observation before being released to the community at large," said the judge.

"The court hopes that your stint (in prison) will help put you in a period of stable remission."

The court was told that Tan believed he had been approached 10 years ago by a Chinese man who told him to pass "data" to the PM.

At about 8pm on June 24, he went to Mr Lee's Meet-the-People Session (MPS) in Ang Mo Kio to hand him an article he had typed. Tan had a black haversack containing a hammer. But Mr Lee did not turn up.

Just before 9pm that night, Tan sent the four threatening messages to Mr Lee's Facebook account.

"Eh, you challenged me to visit your MPS but you are not here. I will find and stage an attack on you when I have information on your public appearances. You know who I am," posted Tan, who included his identity card and mobile phone numbers.

Tan claimed in court that he had been prompted by "people" who had planted surveillance devices in his home. "The people" told him to find Mr Lee and throw a hammer at him.

Tan then checked Mr Lee's Teck Ghee ward Facebook page to find out his next public appearance.

His parents told the police he was a violent person who always carried a knife in his bag. When four officers approached him at his block in Hougang Avenue 1 on June 25, Tan punched one and elbowed another.

A search of his home uncovered a bag containing a hammer, a chopper and several knives. There was also a piece of paper in a bedroom listing Mr Lee's public appearances.

Deputy Public Prosecutor (DPP) Andre Chong urged the court to impose a total sentence of between 18 and 23 months' jail, noting that Tan's own family members were fearful of him. Both his parents had recently applied for personal protection orders, although his mother has withdrawn her application.

The prosecution was not placing any weight on the target of his intimidation charge, and was only considering Tan's "own condition and the risk that he con-tinues to pose to the public", said DPP Chong.

In mitigation, Tan's pro-bono lawyer, Ms Alice Tan, said he was not a dangerous offender and that the key sentencing consideration should be his rehabilitation. She asked for a total sentence of 11 months' jail.

Tan was an "exceptionally bright child", scoring three A*s and an A in his Primary School Leaving Examination. But when he was 15, he could not keep up with his schoolmates and was transferred from the Special to the Express stream. It was around that time that he started hearing "microphones in the walls at home that spoke to him".

He took the N levels as a private candidate, and later did odd jobs in supermarkets and in the food and beverage industry.

Tan twice saw IMH doctors, in 1999 and in 2006. He was found to have "conduct disorder" but "no mental illness". He later defaulted on his follow-up treatment plans.