On Thursday, 37-year-old William Clark was arrested near another downtown subway station and charged with aggravated assault, among other charges. In 1997 Clark was sentenced to seven and a half to 15 years in prison for a series of gunpoint robberies. In 2009 he was accused of breaking into his wife's home and making death threats against her; the District Attorney's Office didn't pursue the case.

Arresting officers said that at the time of his arrest last week Clark appeared mentally ill; officers who work Philly's downtown have extensive contact with the city's substantial homeless population and are trained on interactions with the mentally ill. The courts have ordered a psychiatric evaluation; the results are pending. Clark is held in the Detention Center, the county jail that houses the system's psychiatric hospital, awaiting a preliminary hearing scheduled early next month.

The chilling episode echoes recent events in New York, where in December a homeless man, Naeem Davis, 30, pushed 58-year-old Ki-Suck Han in front of an oncoming train. Later in the month, Erika Melendez, a mentally ill woman from Queens, similarly pushed 46-year-old Sunando Sen to his death.

Philadelphia is no stranger to high profile subway attacks by mentally ill offenders. In 2008 Thomas Scantling, who at the time was not taking medication to treat his schizophrenia and who compounded his mental health problems by abusing PCP, attacked 20-year-old Dewayne Taylor on the subway as Taylor was nodding off listening to his iPod. The attack was so shocking because Scantling, who was in the company of his 6-year-old son, is vividly captured on video pulling a hammer from his bag and using it to pound Taylor repeatedly in the head. He violently drags Taylor from the train car and onto the subway platform, where he continues the attack.

Scantling, who had an arrest record that included rape and drug charges, was in the midst of a paranoid delusion and thought Taylor was out to get him. He was given 10 years in jail followed by 15 years of probation.

"This was about you wanting to take drugs, along with the mental illness," a judge told Scantling at his sentencing hearing. "You're going to be watched for a long time to make sure this doesn't happen again."

Taylor came away from the attack with non-fatal head wounds, but the emotional trauma of the attack lingered.

Around the same time as Scantling's subway hammer attack, Philadelphia rolled out its criminal mental health court. Designed to steer low level offenders towards outpatient mental health treatment instead of county jail, the program claims to have saved tens of thousands of cumulative days in jail time and millions of dollars in its three years of existence by ensuring lawful behavior through compulsory therapy and medication management. Advocates of mental health courts say they can prevent terrifying high profile violence of the sort described here by catching mentally ill offenders early and providing them with supportive services.