This is part of a FOXNews.com series of looking at some of the most wanted criminals being sought by U.S. authorities.

To neighbors in Gary, Ind., Randy Mark Yager was a nice guy, always eager to lend a helping hand.

But to the Outlaw Motorcycle Club, "Mad" was a vicious leader who was feared by his enemies and allies alike.

And to the U.S. Marshalls office, he is one of America’s 15 most wanted fugitives.

As regional president of the Outlaws, Yager was one of 17 members indicted in a massive racketeering conspiracy case that included murder, arson and use of explosives, among a long list of charges.

"A lot of it was from an ongoing feud that the Outlaws had with a subgroup of the Hells Angels, the Hells Henchmen," said Deputy U.S. Marshal Patrick Amerson. "They had bombed the Hells Henchmen clubhouse on the north side of Chicago, they had planted incendiary devices on several of known Hells Henchmen’s vehicles. …They were also accused of what started out as a robbery and turned into the murder of an elderly couple in Northern Illinois."

Federal agents investigated the Outlaws for well over 18 months and found that Yager was considered one of their most violent members.

"One of the positions they have in the club is a sergeant-at-arms, and if there was any corporal punishment, the sergeant-at-arms was normally the one that would carry that out," Amerson said. "But when Yager was the president, he took relish in doing that himself. A lot of the guys were scared because they thought he was just crazy."

But in June 1997, when the Bureau of Alcohol, Tobacco and Firearms made raids and arrests in the case, Yager avoided the sweep.

"ATF went to his home address, couldn’t find him there, then some other places in Indiana. They found out later that he was in Las Vegas, Nevada, with a girl named Margaret Jelovcic," Amerson said.

• Click here to see photos of Yager and Jelovcic.

The fluke trip allowed Yager to become the only Outlaw who was not arrested in the indictment, and it began his life on the lam.

Jelovcic returned to Gary, but not for long.

"Margaret came back for about two and a half, three months, she liquidated all of his assets from the used car lot he owned and then took off," Amerson said.

The then 30-year-old refused to talk to the police during her stay. Whether she was an accomplice helping Yager evade arrest remains unknown.

"Friend told us later, after she’d left, that she was afraid for her life, that’s why she wouldn’t cooperate at all. As far as bringing criminal charges, we never got to that point — couldn’t really prove anything." Amerson said.

"Her family says no way she’d be gone this long without being held against her will. But I have nothing to lead me to believe that."

Jelovcic has been listed as a missing person; Neither she nor Yager has been found.

Some officials believe Yager has been able to hide for so long because he had been preparing for life on the run for quite some time.

"There were rumors that the senior members of the gang put together false identification and had been funneling club dues into their own accounts, so that when the time came and the arrest warrants were finally served, they were able to grab their false ID — they had their slush fund — and hit the road," Amerson said.

Authorities also believe that the motorcycle club continues to play a part in Yager's run from justice.

"I think that he’s still being assisted and aided by the Outlaws, and he probably still has a say," Amerson said.

Randy Yager is 5-foot-11 and 235 pounds. He has hazel eyes, brown hair, and usually wears a mustache, goatee or beard. He has numerous tattoos, including a Harley-Davidson logo on his left upper arm, Outlaw Indians on his lower left arm, an eagle on his right arm and a scar on his abdomen.

Fellow bikers have described Yager as looking Hispanic. They say he likes to drink Crown Royal whiskey and Corona beer. They also say he is arrogant and likes to be "large and in charge."

But his violent nature is not always apparent.

"His neighbors said he was the nicest guy you could ever want, that he’d come over and help you with your car if it broke down. They’d offer him money and he wouldn’t take it. They said he was a decent guy that way," Amerson said.

The U.S. Marshals consider him armed and dangerous and are offering a cash reward for information leading to his arrest.

Authorities suspect Margie Jelovcic is traveling with Yager. She is described as 5-foot-3, 140 pounds, with red hair and green eyes.

Anyone seeing Yager or Jelovcic should call their local U.S. Marshals office, or call Marshals Headquarters at 1-800-336-0102.

Click here to see Randy Yager's U.S. Marshals Wanted Poster.

Click here for more coverage from America's Most Wanted.