Two members of the New Black Panther Party who were arrested last week on federal weapons charges are alleged to have had plans to bomb St Louis' iconic Gateway Arch, and kill a prosecutor and police chief involved in the Michael Brown shooting case, according to reports.

The men planned to kill St. Louis County prosecuting attorney Robert McCulloch and Ferguson police chief Thomas Jackson, according to sources cited by the St Louis Post-Dispatch. The report added that the pair aimed to plant an explosive device on the observation deck of the Gateway Arch. Law enforcement sources told the paper they were unsure whether the two men had the capability to carry out the attacks, and smuggle bombs past the Gateway Arch's airport-style security.

The pair were arrested Nov. 20, and charged with the illegal purchase of two Hi-Point .45 ACP pistols, and purchasing what they thought to be explosives during an undercover FBI sting operation, according to CBS News. Additional charges in connection with the alleged plot are to be added to those they already face, according to the Post-Dispatch.

Their arrest came three days before the decision of a Ferguson grand jury not to indict Officer Darren Wilson over the fatal Aug. 9 shooting of Michael Brown was made public. The two men were under investigation for months and had been aggressively pursuing the acquisition of explosives, according to sources cited by KDSK News.

Waves of protests have gripped Ferguson, Missouri, and many other cities and towns across the U.S., in the wake of the decision not to indict Wilson.

President Barack Obama had called for calm in the wake of the grand jury's decision. “This is not just an issue for Ferguson – this is an issue for America,” he had said, adding, “There are still problems and communities of color are not just making these problems up... These are real issues - we can’t deny them.”