Investigators believe that the person who killed two Brown University students this past Saturday may have cased neighborhoods near the campus hours before the shooting, as police continue their search more than three days after the attack.

The Providence, Rhode Island, Police Department released enhanced security camera footage Tuesday showing a man walking through residential areas near the university earlier on the day of the shooting, which occurred around 4 p.m. in an engineering building on campus.

The New York Times reported that the images show the individual circulating near campus as early as 10:30 a.m., several hours before the attack, raising the possibility that he was surveying the area in advance.

"We believe he was actually casing out this area, which is something that criminals do," said Col. Oscar Perez, Providence police chief.

Police said the culprit's face is covered in all of the released images, limiting their ability to identify him. Investigators urged the public to focus instead on the shooter's body movements, posture, and gait.

"Focus on the body movements, the way the person moved their arms, the body posture, the way they carry the weight," Perez said. "Those are important movement patterns that may help you identify this individual."

Rhode Island Attorney General Peter Neronha said detectives do not yet have a suspect or a motive.

"We don't yet know who we're looking for," Neronha said. "Once we identify who this person is, I believe we'll be able to locate him.

"It's very hard to hide in this country."

The FBI has also released a compilation of video footage, much of it from home security cameras, showing a man walking, loitering, and running in residential areas near campus before and shortly after the shooting.

Investigators said no usable video was captured in the building where the shooting occurred. The building, Barus and Holley, has been renovated in recent years, but the shooting took place in an older section with fewer cameras.

"There is no footage that depicts this individual that would be useful in identifying him that we have not released," Neronha said.

The shooting killed two students and injured nine others during an economics review session.

Loved ones continued to grieve the loss of the two students killed in the shooting.

Mukhammad Aziz Umurzokov, 18, was from Virginia and came to the United States with his family from Uzbekistan in 2011.

Ella Cook, 19, a sophomore from Alabama, was known as a talented pianist and served as vice president of the university's Republican Club.

Police have said they do not believe anyone else was involved in carrying out the attack.

One individual described as a person of interest was detained Sunday but later released.

Security around the campus has been increased, and Brown University has canceled all remaining classes and examinations for the fall semester.