LAS VEGAS — Still unable to identify a motive five days after the massacre of 58 concertgoers, local and federal officials sounded increasingly desperate for leads on Friday, announcing plans to erect billboards with the message, “If you know something, say something.”

“There are still a number of people out there that know that something looked out of place,” said Undersheriff Kevin McMahill of the Las Vegas Metropolitan Police Department. “Someone may have been acting suspiciously that night, or in the years prior, the months prior. Someone that may have seen something or knows something.”

Aaron Rouse, the special agent in charge of the F.B.I. office in Las Vegas, said billboards would be set up around Las Vegas area, to “reach as many people as we possibly can,” seeking tips about the gunman. “We have not stopped, we will not stop, until we have the truth.”

• Though the gunman, Stephen Paddock, had explosives and chemicals that can be used in bomb-making in his car, “it did not resemble an I.E.D.,” or improvised explosive device, Undersheriff McMahill said. Later, speaking on CNN, he clarified, “I believe he certainly had nefarious intent with that material,” but he did not know what that intent was.