Federal authorities in Denver opened an investigation after police found rifles and ammunition in a rental truck days before Democratic presidential candidate Barack Obama was set to accept his party`s nomination, according to Bloomberg.

Three men identified as white supremacists were arrested on the morning of Aug. 24 after a routine traffic stop that yielded the weapons, a bulletproof vest, walkie-talkies and illegal drugs, officials said. All three had arrest records.

Denver`s CBS television affiliate, citing law-enforcement sources, reported that one of the suspects indicated that the men had come to Denver to assassinate Obama, a 47-year-old Illinois senator.

Federal authorities were notified because of the suspicious circumstances and heightened security for the Democratic National Convention, said Detective Marcus Dudley, a spokesman for the Aurora, Colorado, police department. ``A reasonable person would understand the nature of the concerns,`` Dudley said.

He declined to comment on whether authorities were investigating an assassination plot. He referred that question to federal officials, who have scheduled a news conference for later today to discuss the circumstances surrounding the arrests.

The truck`s driver, Tharin Robert Gartrell, 28, was on probation for previous drug charges and was driving with a suspended license, Dudley said. He was charged with drug possession and material to make methamphetamines.

Hotel Arrest

Nathan Johnson, 32, was arrested by local and federal authorities at a Denver hotel near Interstate 25, which runs past Invesco Field, where Obama is scheduled to give his acceptance speech Aug. 28, Dudley said.

Shawn Robert Adolf, 33, was held on seven outstanding warrants. He jumped out a sixth-floor hotel room in Glendale as police entered and was later taken into custody, Dudley said. Adolf suffered a broken ankle from the fall.

The three belonged to the white supremacist group Aryan Nation, said Victor Ross, police chief in Glendale, a suburb of Denver where one of the suspects was arrested. The other two were arrested in Aurora, another suburb.

``We don`t usually have that number of Aryan Nation people here,`` Ross said. ``It was suspicious to us. We turned everything over to the federal authorities.``

`No Credible Threat`

FBI spokeswoman Kathleen Wright said federal authorities had opened a drugs and weapons investigation in connection with the arrests. The U.S. attorney`s office in Denver said in a brief statement that federal charges were anticipated.

Troy Eid, the U.S. attorney for Colorado, said ``federal law enforcement is working hand-in-glove with the Aurora Police Department.``

He added: ``We`re absolutely confident there is no credible threat to the candidate, the Democratic National Convention, or the people of Colorado.``

The rifle with a scope was reported stolen from Goodland, Kansas, Dudley said. The rifles were found along with a quantity of methamphetamines in the rental truck, which had not been leased to the driver, Dudley said.

Dudley said none of the three suspects in custody had a fixed home address, which was ``another cause for alarm.`` One was being held on outstanding warrants with bond worth more than $1 million.

Obama spokesman Bill Burton declined comment. ``We don`t talk about security issues,`` he said.

The incident was the second this month involving a threat to Obama, who is seeking to become the first black president.

Raymond Hunter Geisel, 22, was arrested in Miami Aug. 2 and charged with threatening to kill Obama, according to the U.S. Secret Service and court records. Geisel allegedly made the threat during a class he was taking to become a bail bondsman, and a search of his truck and his hotel room turned up weapons.

Obama has been receiving Secret Service protection since May 2007, the earliest date that any presidential candidate has gotten a security detail since the practice was instituted following the 1968 assassination of Senator Robert F. Kennedy.