‘We’re going to go until we’re absolutely satisfied that we’ve uncovered every bit of evidence’ on Omar Mateen, US attorney for Florida said on Wednesday

Families of victims of the gay nightclub massacre in Orlando could be forced to wait years for justice as detectives interview hundreds of people and comb through vast hoards of digital media.

The FBI has published a “seeking information” poster with four pictures of gunman Omar Mateen, 29, appealing for the public to come forward with leads and tips. Its investigation will extend deep into the American-born Muslim’s past and is not expected to conclude any time soon.

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“We’re going to go until we’re absolutely satisfied that we’ve uncovered every bit of evidence that’s out there and we’ve reviewed every possible charge,” Lee Bentley, US attorney for the middle district of Florida, told reporters on Wednesday. “That’s when this investigation will end. It might be a matter of days, it could be weeks, it could be years. There’s simply no way to determine at this time how long the investigation will last.”

Investigators are talking to “literally hundreds of people” who might have crossed paths with Mateen in recent years and be able to offer a clue as to his actions, Bentley added. “We are investigating not only this crime but law enforcement is talking to everyone associated with the shooter and that includes his family, his friends, people in businesses. It includes anyone who fell within the ambit of what the shooter was doing in the months leading up to the crime.”

He repeatedly declined to comment on reports that Mateen’s widow, Noor Zahi Salman, could soon be charged over allegations that she knew of the killer’s plot but failed to warn police or do anything to stop him.

The FBI investigation is both broadly sweeping and intensely focused on detail, which is likely to frustrate anyone expecting instant answers. Four days after the atrocity, the FBI’s evidence response team are still sweeping Pulse nightclub with work that includes analysis of the trajectory of the bullets.

The investigation also includes an attempt to reach far into Mateen’s personal history and motivations. Questions include whether he was radicalised by the internet or had direct contact with Islamist miltant groups and claims that he used gay dating apps to chat to men.

Ronald Hopper, the FBI assistant special agent in charge, said: “The FBI is reconstructing the subject’s movements going back months, day, hours and minutes before the fatal act here at the Pulse nightclub and that is something we are looking at and scrubbing thoroughly.”

Mateen was questioned in 2013 and 2014 by the FBI but this resulted in no further action being taken. Hopper added: “We are looking at everything he did leading up to this attack and we are going backwards as well to look at all of the things we knew about before and things that we’ve investigated in the past. This is an investigation that has no finite end at this particular time and that goes from the beginning. We’re going to go back even further than we did if that’s possible.”

The inquiry includes foraging through “voluminous amounts” of digital media, Hopper said. So far, there is nothing to suggest there was any target other than the Pulse nightclub, nor to confirm reports that Mateen had visited other clubs.

The FBI official promised: “We will leave no stone unturned, and what that means is, at the end of all of our interviews, however long that takes, if someone is able to be charged in this investigation, we will bring them to justice.”

Buddy Dyer, the mayor of Orlando, said Mateen drove around the city on Saturday evening, going from one place to another, before he stormed Pulse and opened fire at about 2am on Sunday. The attack ended with Mateen being killed by a Swat team.

“What I know concretely is that he was driving around that evening and visited several locations,” Dyer said.

During the hostage standoff, Mateen told police negotiators that he planned to strap bombs to four people, Dyer added. In phone calls and text messages, people trapped inside the club, who could hear what the killer was saying, also raised the alarm over explosives. “We had independent verification of that,” Dyer said. “We had a lot of information from the inside and they independently were saying yes, the bomber is about to put on an explosive vest.”

An Orlando TV station said on Wednesday that Mateen called in during the standoff with police and declared his support for the Islamic State. Matt Gentili, a producer at CFN13 in Orlando, said at about 2.45am a man phoned him and said: “I’m the shooter. It’s me. I am the shooter.”

Salman and Mateen married in 2011 and have a three-year-old son. The Associated Press reported that Jessie Rojas, a nextdoor neighbour, said three people identifying themselves as FBI agents visited Salman’s childhood home in Rodeo, California, on Tuesday and spoke with her mother. Rodeo is in the dry hills near the oil refineries 25 miles north-east of San Francisco.

On Wednesday, a film clip of Mateen came to light from The Big Fix, a 2012 documentary about the BP oil spill in the Gulf of Mexico. It shows him working as a security guard at a cleanup site in Florida and telling a woman who pulls up to his guard booth that everyone is “hoping for more oil to come out and more people to complain so they’ll have the jobs”.

Orlando’s police chief, John Mina, said police officers had received a critical stress management debriefing. The officers, “some of the toughest men I know”, had “stood face to face, toe to toe, with a mass murderer and I am proud of them”, he said.

Meanwhile Orlando is preparing for a visit by Barack Obama on Thursday. The White House press secretary, Josh Earnest, said the president will meet and offer condolences to the families of those who were killed and will seek to comfort those who survived, as well as first responders, doctors and nurses.