Charleston is pumping more money into efforts to catch a serial arsonist who has eluded authorities while setting a string of fires over a decade across the central peninsula.

Mayor Joe Riley announced Thursday that the city is doubling its reward -- from $25,000 to $50,000 -- for anyone whose tip helps find and convict those responsible.

Riley, who has been mayor for 36 years, said he could not remember an instance when the city put up as large a sum to try to solve a crime. Typically, family members, businesses or other parties put up a reward.

The city announced a $25,000 reward in late July, after a suspicious early morning fire occurred at a two-story home on Cannon Street.

Riley said the idea to double that amount came from the city's arson task force.

Police Chief Greg Mullen said the city has received tips, "but what we're trying to do is make sure that we are making the incentive large enough for people to come forward and provide us with the necessary information we need. ... Increasing the reward money at this time might offer that incentive."

"We believe there are good citizens in our community who have information that could be helpful," Riley added. "We encourage anyone who has any information, or even if you just think you have information that might be in the back part of your memory ... don't be timid. Please provide that information."

The $50,000 sum comprises $45,000 in city money and $5,000 from the S.C. Insurance News Service.

Riley said the higher reward is not related to the city's current interest in finding a man who police want to interview regarding an early Saturday morning fire that erupted in recycling bins inside 54 Vanderhorst St., he said.

The city has released a sketch of a man in his early 20s who warned a resident there about the fire, helped douse it, then left.

"This is a person is a witness, not a suspect," Fire Department spokesman Mark Ruppel said.

It is unclear if the Saturday morning fire is the work of the downtown serial arsonist.

The fire occurred at a four-story wooden house that contains four apartments rented mainly to students. A resident said she and her roommates were sleeping when someone knocked on her door at 4 a.m. and pointed out the still-smoldering fire in the recycling bins next to her door.

Two of the three recycling bins burned completely, while a third melted. Flames blackened the ground-level porch floor and a nearby wall. Police weren't notified of the fire until several hours after it was put out.

Saturday's fire fit the pattern of suspicious blazes because it occurred between midnight and 5 a.m., but it differed slightly in location. Vanderhorst Street is a few blocks south of where most fires were intentionally set.

At least 85 suspicious fires have broken out north and south of the Septima P. Clark Parkway since 2000, according to The Post and Courier's analysis of police and fire reports. Most were set between midnight and 5 a.m. in porch areas and at homes rented by young people.

Riley and Mullen declined to say anything about the details of the city's investigation other than to say it is ongoing.

Charleston officials want to interview a witness to a Saturday morning fire at 54 Vanderhorst St. The man is described as about 5-feet-10 with a medium build, blond hair and blue eyes, and was last seen wearing a gray sweater and white tennis shoes. He is asked to call police or fire officials at 577-7434.

Anyone who might have information about the arsons should call 1-800-92ARSON (1-800-922-7766).

The city of Charleston offered these tips as it tries to solve what's behind a string of fires across the central peninsula:

Stay vigilant. Know your neighbor.

Call 911 if you see anything suspicious happening in your neighborhood.

Report incidents right away. Do not wait.

Don't become a target. Remove easily combustible material from your porch area and leave your porch light on.

Make sure you have a working smoke alarm and test it monthly. Change its batteries at least once a year.