DENVER — City and federal authorities said Monday night that at least four people were under arrest on firearms and drug charges in connection with a possible plot to kill Sen. Barack Obama during his speech accepting the Democratic presidential nomination on Thursday night.

The U.S. attorney's office in Denver has scheduled a news conference today to discuss its investigation. A spokesman for the Denver Police Department said that an "aggressive" investigation had been under way since the first arrest in the case. That occurred during a traffic stop in Aurora, Colo., on Sunday morning, when the police seized firearms and methamphetamine, the authorities said.

It was not clear how well the possible plot against Obama had been planned, or whether those arrested had found a way to overcome the heavy security surrounding the Democratic convention. Obama is scheduled to deliver his speech at Invesco Field, near the convention center.

The CBS affiliate in Denver reported on Monday night that the police had found two high-powered rifles with scopes in the car, along with camouflage clothing, walkie-talkies, a bulletproof vest, a spotting scope and licenses in the names of other people. One of the rifles was listed as stolen from Kansas.

The investigation — which included the Secret Service, the FBI and a joint terrorism task force, officials said — was triggered by the initial arrest on Sunday morning.

"The matter is being taken seriously," the spokesman said before referring additional questions to the U.S. attorney's office.