BOSTON -- Police released a new sketch Thursday of the man officials believe is responsible for two sexual assaults near the Charles River in Boston last year.

The new sketch is based on a description provided to members of the Boston Police Sexual Assault Unit by the victim of a July 29, 2007, attack. Forensic evidence obtained by Massachusetts State Police links that assailant to two other sexual assaults -- one on June 16, 2007, also near the Esplanade, and one at Moakley Park in South Boston on July 29, 2006.

Officials urged any witnesses to the incidents to come forward. They reminded pedestrians to remain aware of their surroundings, especially in isolated areas.

"Every branch of law enforcement is committed to catching this predator," Suffolk County District Attorney Daniel F. Conley said. "Our efforts depend on witnesses and their observations, and we urge anyone who may have seen or heard something related to these attacks to come forward. Your statements could help us crack this case."

"We hope that the release of this sketch will generate new details, which will provide our detectives with the critical investigative leads necessary to capture this sexual predator," Boston Police Department Commissioner Ed Davis said.

In light of warmer temperatures, authorities also reminded members of the public to remain alert and aware of their surroundings -- especially in isolated areas late at night or early in the morning, when the Esplanade and South Boston attacks took place.

Authorities last year sought five individuals who may have made observations that could prove valuable in bringing the at-large suspect to justice. They renewed their call for certain witnesses -- or anyone else with knowledge of the assaults -- to come forward.

Specifically, investigators would like to speak with:

A man who was present near one of the Esplanade footbridges at about 11:15 p.m. on June 16, 2007. That individual saw the victim and the assailant.

A pair of bicyclists wearing helmets and cycling gear who were walking their bikes near the footbridges between 4:30 a.m. and 5 a.m. on July 29, 2007. They saw the assailant and the second victim, prompting the assailant to put his arm around the woman as if they were a couple.

A male and female, both well-dressed and in their late 20s, who were near Joe's American Bar and Grill at the intersection of Dartmouth and Newbury streets between 5 a.m. and 5:30 a.m. on July 29, 2007.

In the July 29, 2007, incident, a 20-year-old woman was abducted at knifepoint on Beacon Street and forced to walk to the Esplanade, where she was sexually assaulted and robbed at about 3:30 a.m.

In the June 16, 2007, incident, a different 20-year-old woman was jogging at the Charles River Esplanade on a footpath near the Massachusetts Avenue Bridge shortly after 11 p.m. when she was sexually assaulted.

In the July 29, 2006, incident, a 30-year-old woman was in the area of Joe Moakley Park when she was sexually assaulted at about 3 a.m.

Each victim described her assailant as a clean-shaven African-American male in his mid-20s to mid-30s, standing 5 feet 8 inches to 5 feet 10 inches. At the time of the attacks, he had a medium to large build and a bald head.

Anyone with knowledge of the crimes or the suspect, or who was present near the Esplanade, Dartmouth Street, or Moakley Park areas at the times noted above, is urged to contact the Massachusetts State Police Barracks in Boston at 617-727-6780, call the toll-free Boston Police CrimeStoppers tip line at 1-800-494-TIPS, or text the word "tip" to CRIME (27463).