By Carey Gillam

(Reuters) - Rumors that a black teenager killed by St. Louis police was only 13 years old helped stoke angry clashes between police and protesters, a city alderman said on Thursday, and police said they were ready should protests flare anew.City streets were largely calm a day after Wednesday's fatal shooting during a house search triggered the protests, but St. Louis Alderman Antonio French said it was hard to say if protesters would take to the streets again Thursday night.

Police said additional resources would be deployed if necessary but gave no details.

French said the protests had been driven partly by talk that the teen killed was only 13. Small groups typically stage peaceful demonstrations nightly in Ferguson but others, not part of organized groups, can be unpredictable, he said, adding:

"It's hard to say right now about what the mood is. There are some folks who are angry ... just looking for a battle."

"There was a lot of misinformation out there. Many arrived angry because they heard a 13-year-old kid was killed," said French, a prominent voice in the community since police in the St. Louis suburb of Ferguson killed an unarmed black teenager last year and sparked months of sometimes violent protests.

Mistrust of police accounts of Wednesday's shooting also helped stoke the unrest, which prompted police in riot gear to fire tear gas to disperse protesters after they threw bricks and bottles at officers.

Police identified the dead man as 18-year-old Mansur Ball-Bey and said he and another African-American man were attempting to flee a home where police were executing a search warrant when Ball-Bey pointed a gun at officers. Police said two officers then fired, hitting him four times and killing him. They said drugs and stolen guns were found in the house.

Some protesters expressed doubt over the police account of Ball-Bey pointing a gun. St. Louis police do not wear body cameras, but St. Louis Police Chief Sam Dotson said some officers videotaped the protests, during which shots were heard.

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Dotson said a car was torched, some businesses reported robbery attempts and nine people were arrested.

The city has been on edge since a white police officer in the mostly black Ferguson community shot and killed 18-year-old Michael Brown on Aug. 9, 2014. Brown's death was one of a series of police killings of unarmed black men and teens across the United States that sparked a newly energized civil-rights movement under the banner "Black Lives Matter".

Debate over largely white police treatment of minorities has become a hot-button political and social issue in America.

Republican senator and presidential candidate Marco Rubio on Thursday said the issue cannot be ignored.

"It’s a reality that in many communities in this country the relationship between minority communities and the police and law enforcement agencies is terrible," he told the Detroit Economic Club.

Police need to do more to allow for peaceful protests rather than escalate them into violence, said Amnesty International USA senior campaigner Jamira Burley, who was critical of St. Louis police actions taken Wednesday.

Ferguson and St. Louis County police came under heavy criticism for their handling of the Ferguson protests, which included use of riot gear, tear gas and heavy armored vehicles, tactics many in the community said only inflamed the situation.

Images of heavily armed police training weapons on citizens prompted U.S. President Barack Obama to tighten restrictions on police use of military equipment.

(Reporting by Scott Malone in Boston and Carey Gillam in Kansas City; Additional reporting by Ben Klayman in Detroit; Editing by Lisa Lambert and James Dalgleish)