(Adds jet fuel storage tank struck by gunfire; details of how gunman was located, paragraphs 5-6 & 18-20)
By Alexandria Sage and Sharon Bernstein
LAS VEGAS, Oct 6 (Reuters) - Baffled police and FBI agents, still lacking a clear motive for the Las Vegas massacre of 58 people by a lone gunman five days ago, appealed to the public on Friday to come forward with any information that might help solve the mystery.
Clark County Undersheriff Kevin McMahill said investigators remain largely in the dark after running down more than 1,000 leads seeking clues to what drove a wealthy retiree with a penchant for gambling to carry out the deadliest mass shooting in modern U.S. history.
The gunman, Stephen Paddock, 64, poured a barrage of gunfire from the windows of his 32nd-floor hotel suite into a crowd of 20,000 people attending an outdoor music festival on Sunday night, then killed himself before police stormed his room.
In addition to the 58 people killed, nearly 500 were injured, some by gunfire, some trampled or otherwise hurt while running for cover.
In a new disclosure, authorities said two bullets Paddock fired struck a large jet fuel storage tank at the edge of the city's main airport, about a block from the concert grounds, indicating an apparent attempt by the gunman to create even greater havoc.
There was no explosion or fire from the two rounds, one of which penetrated the tank, as jet fuel in storage is almost impossible to ignite with gunshots, airport officials said on Friday.
Unlike so many other perpetrators of deadly mass shootings before him, Paddock left behind no suicide note, no manifesto, no recordings and no messages on social media pointing to his intent, according to police.
"We have looked at everything, literally, to include the suspect's personal life, any political affiliation, his social behaviors, economic situation, any potential radicalization," McMahill told reporters. "We are looking at every aspect from birth to death of this suspect and this case."
McMahill acknowledged that Islamic State had repeatedly claimed responsibility for the attack, but said investigators had uncovered "no nexus" between that Mideast-based militant group and Paddock.
In an unusual bid to cast a wider net for additional tips, the FBI and police have arranged with communications company Clear Channel to post billboards around Las Vegas urging members of the public to come forward with any information they believe might help investigators.
The billboards will bear the slogan, "If you know something, say something," and carry a toll-free number to an FBI hotline, said Aaron Rouse, special agent in charge of the Las Vegas FBI office.
"We have not stopped, we will not stop until we have the truth," Rouse said.
McMahill said investigators were satisfied that no one else was in the room with Paddock, who checked into the Mandalay Bay hotel three days before the massacre.
"We're very confident ... there was not another shooter in that room," he said.
But police have said they suspect Paddock may have had assistance at some point before the killings, based on the large number of guns, ammunition and explosives that were found in the hotel suite, his home, his car and a second home searched in Reno.
Authorities have said that 12 of the weapons recovered from Paddock's hotel suite were equipped with so-called bump-stock devices that enable semi-automatic rifles to be operated as if they were fully automatic machine-guns.
Paddock's ability to fire hundreds of rounds per minute over the course of his 10-minute shooting spree was a major factor in the high casualty count, police said.
The bloodshed might have lasted longer, with greater loss of life, but for a hotel security officer who was sent to check an open-door alarm on the 32nd floor, and discovered the gunman's whereabouts after the shooting started, McMahill said.
The security officer, Jesus Campos, was struck in the leg as the gunman strafed the hallway with gunfire from behind his door, apparently having detected Campos via surveillance cameras Paddock set up outside his hotel suite.
Campos, though wounded, alerted the hotel's dispatch, "which was absolutely critical to us knowing the location as well as advising the responding officers as they arrived on that 32nd floor," McMahill said. "He's an absolute hero."
Reports have emerged in recent days that Paddock may have targeted other sites for attack in Las Vegas, Chicago or Boston, which police have said they were investigating.
Paddock's girlfriend, Marilou Danley, 62, was questioned by the FBI on Wednesday and said in a statement she never had any inkling of Paddock's plans.
Danley, who returned late on Tuesday from a family visit to the Philippines, is regarded by investigators as a "person of interest." The Australian citizen of Filipino heritage is cooperating fully with authorities, her lawyer said.
Reporting by Alexandria Sage and Sharon Bernstein in Las Vegas; additional reporting by Susan Heavey, Richard Cowan, Doina Chiacu, Amanda Becker and Jeff Mason in Washington, Chris Kenning in Chicago, Karen Freifeld and Jonathan Allen in New York, Keith Coffman in Denver and Brendan O'Brien in Milwaukee; Writing by Scott Malone and Steve Gorman; Editing by Janet Lawrence and Andrew Hay