(Adds reaction from St. Louis County police, Anonymous group)
By Nick Carey
FERGUSON, Mo., Aug 14 (Reuters) - Authorities in Missouri on Thursday stood by their earlier decision to withhold the name of the police officer who fatally shot an unarmed black teenager, but denied he was the person identified online by a hacker activist.
Demonstrators and the family of the victim, 18-year-old Michael Brown, have called on police to release the name of the officer, but officials have refused to do so, citing security concerns in a tense environment amid days of protests that have sometimes turned violent.
Among the concerns, they said, are online threats from the hacker group Anonymous, which has said it would release personal information about the police officer involved and on Thursday identified him by name in a Twitter post.
The St. Louis County Police Department tweeted that the name given by Anonymous was of a man who is not an officer with the Ferguson police or the neighboring St. Louis County police.
"Do not release more info on this random citizen," said a tweet from @stlcountypd.
"We can't let anonymous groups or even public groups pressure us into doing anything we don't think we should do," Edward Magee, spokesman for the St. Louis County prosecutor's office, told Reuters.
A member of Anonymous, who goes by @The Anon Message on Twitter, told Reuters in an email that it was standing by the name it identified. "Of course they'll deny," the person said, referring to the police in Missouri.
In Ferguson, protesters have gathered every night since Brown was fatally shot on Saturday in the mostly black suburb of St. Louis during what authorities said was a struggle over a gun in a police car. Some witnesses say he was outside the car with his hands up.
Police in Ferguson fired tear gas, stun grenades and smoke bombs on Wednesday night to disperse some 350 protesters, the fourth night of racially charged demonstrations.
Missouri Governor Jay Nixon was scheduled to visit north St. Louis County on Thursday to assess the situation, and his office said he would also make an announcement. The governor has urged law enforcement agencies to respect the rights of the residents and the press.
Dozens of protesters have been arrested since Saturday. On Wednesday, two journalists, Wesley Lowery of the Washington Post and Ryan J. Reilly of Huffing ton Post, were arrested and then released, while working in the area.
Protesters have said the lack of transparency by police investigating the incident -- including the refusal to release the officer's name -- have added to already high tensions. They have also called for St. Louis County Prosecuting Attorney Bob McCullough to be removed from the case.
Magee said the officer's name will be made public if he is charged. Even if he is not arrested, his name will be released after the investigation because of the high public interest, Magee added. The officer has been placed on administrative leave.
Anonymous, which has also called for nationwide protests over the situation in Ferguson, said it would release the officer's photo and other "damaging information" later Thursday.
Police have deployed camouflage-clad officers in body armor, including one manning a rifle on a tripod atop an armored car, to Ferguson.
"I've had enough of being pushed around because of the color of my skin. I'm sick of this police brutality," said one protester, who gave only his first name, Terrell, 18. "I'm going to keep coming back here night after night until we get justice."
National figures from President Barack Obama to civil rights activist Rev. Al Sharp ton have called for a peaceful response to the shooting.
Police Chief Thomas Jackson told reporters his priority was improving race relations in Ferguson, which has seen a stark demographic shift in recent decades, going from mostly white to mostly black. About two-thirds of the town's 21,000-strong population are black. On a police force of 53, three officers are black.
"This is an opportunity to fix what's wrong," he said at a news conference on Wednesday. (Additional reporting by Brendan O'Brien in Milwaukee and Carey Gillam in Kansas City; Editing by Susan Heavey)