Transit police arrested a Sudanese diplomat on Monday afternoon after observing him rub his crotch against a woman on the 4 train near Grand Central, according to the NYPD. Initial charges of sexual assault and forceable touching were dropped, however, when police confirmed 49-year-old Mohammad Abdalla Ali's diplomat status.

Ali, a finance attaché for the Sudanese Mission to the United Nations, benefited from diplomatic immunity—a policy that has protected diplomats from fines as innocuous as parking tickets and charges serious as rape in their host countries.

According to the NYPD, officers saw Ali "acting in a suspicious manner" behind a 38-year-old woman on a northbound 4 train, shortly after 2:00 p.m. on Monday. The train had just pulled out of Grand Central Terminal. Following Ali's arrest, charges of forceable touching and sexual assault were voided. The case was referred to the State Department.

"We are aware of allegations of a criminal act by a Sudanese diplomat that led to the involvement of the NYPD," said State Department Deputy Spokesperson Mark Toner in a statement. "We are in touch with the NYPD and the Mayor's office so that we are able to take appropriate steps, as needed, to address this matter."

The Mayor's office did not immediately respond to a request for comment; the Sudanese Mission also did not immediately comment.

Since 1978, diplomats have been legally immune from both criminal prosecution and lawsuits pertaining to their activities both on and off the job, according to the Diplomatic Relations Act. Al Jazeera reported on the policy in 2015, which is intended to protect diplomats from unfair trial in hostile countries. It came under fire that fall, when Majed Hassan Ashoor, first secretary at Delhi's Saudi Embassy, invoked the privilege after allegedly beating and raping two Nepali women who worked for him. "The provisions are plainly antiquated and not fit for purpose in a modern era," Schona Jolly, an international human rights lawyer, told the outlet at the time.

Last fall in New York City, a German UN attaché reportedly invoked immunity after allegedly punching his wife in the face.

City Councilmember Rory Lancman introduced legislation in 2014, known as the Diplomatic Immunity Notification Act, that would require the NYPD to notify the City Council and Mayor's office of any incident in which a diplomat benefits from immunity. The council has yet to hold a hearing on the bill.