A female jogger was assaulted on the Charles River Esplanade last Thursday afternoon when a man on a bicycle grabbed her buttocks, the Massachusetts State Police Department said.

A witness chased down the suspect and apprehended him until a trooper arrived, state police Sgt. Matthew Murray said.

The 26-year-old suspect was arrested and charged with indecent assault and battery on a person.

Troopers don't believe that the Thursday assault is related to the unsolved rapes that occurred on the Esplanade between 2006-2009, Murray said.

"The other assault investigations are still open and ongoing and we are still seeking an entirely separate suspect for those assaults," he said.

The Boston University Police Department Chief Thomas Robbins and Dean of Students Kenneth Elmore sent out an email to alert students of the Thursday assault and of the gang-related fight in Allston last weekend.

"These incidents remind us that, although we are in a relatively safe location within the city, we are not immune to crime that takes place in an urban area," the email said.

Despite the assault incident, College of Arts and Sciences junior Alix Kraminitz said that she still feels pretty safe jogging on the Esplanade during the day.

"It's a little bit off-putting, but it's not going to make me stop running," Kraminitz said.