ANN ARBOR, MI - Police are seeking help identifying three suspects they believe spray painted hateful graffiti in an alley.

The suspects, caught on surveillance cameras, are described as three white men in their early 20s, said Ann Arbor Police Detective Lt. Matthew Lige.

The racist graffiti was discovered Sept. 16 in an alley off the 600 block of East Liberty Street. "Free Dylann Roof, I Hate N******" was painted in red over a mural in the alley.

Roof, 23, is a white supremacist and mass murderer who killed nine African Americans at a church in Charleston, South Carolina, in 2015.

Two surveillance videos show the suspects in and near the alley in the early morning hours of Sept. 16.

The first suspect has light-colored hair and wore glasses, a dark-colored T-shirt and tan cargo pants, police say.

Photos were taken from two different cameras, Lige said. One was infrared, which made the the suspect's T-shirt look white, rather than dark.

Police have confirmed it's the same man, Lige said.

The second suspect was wearing a light-colored stocking cap, a dark jacket and pants. He carried a backpack, according to police.

Suspect No. 3 wore a dark-colored sweatshirt and pants, dark-colored hair and eyeglasses on the top of his head, police say. He also carried a backpack.

All were described as 5-foot-9 to 6 feet tall.

Police believe the third suspect wrote the graffiti while the other two kept a lookout, Lige said.

"We are in the process of identifying the individuals involved and ask that anyone with information contact the Ann Arbor Police Department tip line at 734-794-6949 or email TIPS@a2gov.org," he said.

There have been several incidents of racist graffiti in Ann Arbor in recent weeks.

Black students from UM were the target of a racist incident in which the name tags on their West Quad dorm doors were defaced with derogatory language on Sunday, Sept. 17.

On Sept. 1, an anti-Hispanic message was found scrawled on the Rock - a UM landmark on Hill Street and Washtenaw Avenue frequently painted by students.

Swastikas and racial slurs were found spray painted at the skate park at Veteran's Park on Aug. 18.