Police in Ferguson, Missouri committed human rights abuses as they sought to quell mostly peaceful protests that erupted after a white officer killed an unarmed black teenager in August, an international human rights organization said in a report published Friday.

Amnesty International found that U.S. authorities should investigate Ferguson law enforcement for alleged abuses that occurred during weeks of racially charged protests. The demonstrations erupted after Ferguson police officer Darren Wilson shot and killed Michael Brown on Aug. 9.

A grand jury in St. Louis County, where Ferguson is located, is weighing whether Wilson should be charged in Brown's death. Wilson has not spoken publicly about the incident.

Anticipating unrest if Wilson is not charged, Missouri police have been stockpiling riot gear. The grand jury will issue its decision next month.

The Justice Department is already investigating Brown's killing and the Ferguson Police Department’s response. Missouri police have also been brushing up on their knowledge of constitutional rights.

Ferguson law enforcement’s use of rubber bullets, tear gas and heavy military equipment and restrictions placed on peaceful protesters all violated international standards, Amnesty said.

Amnesty said it sent a delegation to Ferguson from Aug. 14 to 22 to monitor the situation.

When asked about the allegations, Brian Schellman, a spokesman for the St. Louis County Police Department, which helped oversee law enforcement operations in Ferguson, said police "had one mission, and that was the preservation of life."

Law enforcement officers expect to receive at least a day's notice before a grand jury announcement. That should provide time for them to execute security plans, but may also allow demonstrators to prepare.

"The moment I learn that there is, in fact, a non-indictment, then there's going to be an organized protest," said Eric Vickers, a St. Louis attorney and civil rights activist.

Police and protesters have repeatedly clashed since the shooting, which prompted a national conversation about race and police tactics. Images of officers in riot gear and armored vehicles confronting protesters have drawn widespread criticism.

The Amnesty report also questions a Missouri law the group said might be unconstitutional because it allows police to use deadly force against someone even if there is no imminent threat of harm.

An officer may use deadly force "in effecting an arrest or in preventing an escape from custody" when that officer "reasonably believes that such use of deadly force is immediately necessary to effect the arrest and also reasonably believes that the person to be arrested ... has committed or attempted to commit a felony."

The report calls on state lawmakers to make Missouri law comply with international standards making lethal force by police a last resort, said Rachel Ward, research director at Amnesty International.

"Lethal force is only to be used to protect life when there is an immediate threat," Ward said. "The Missouri statute goes far beyond that. It is of grave concern."

Witnesses and law enforcement officials have said Brown and Wilson got into an altercation after Wilson told Brown to stop walking down the middle of a street.

Wilson shot Brown six times. Some witnesses reported that Brown had his hands up in surrender when the last shots were fired.

"Michael Brown was unarmed and thus unlikely to have presented a serious threat to the life of the police officer," the report said.

As the grand jury’s decision looms, officers have adjusted their tactics for interacting more peacefully with protesters, while also honing their procedures for quick, widespread arrests.

They plan to have a large contingent of officers at the ready, but have been meeting with clergy, community leaders and students in hopes of building relationships that could ease tensions on the streets.

"I know there's a lot of anxiety, there's a lot of fear, anticipation" about that announcement, said Missouri State Highway Patrol Capt. Ron Johnson, who was put in charge of security in Ferguson in the days after Brown was killed and is now part of a coordinated command with local police.

"I have a lot of hope," despite the potential for unrest, Johnson said.

Wire services