SUNRISE, FLORIDA (NYTIMES) - While investigating a tip late last year about a man suspected of possessing child sexual abuse imagery, deputies from the sheriff's office in Walton county, Florida, worried that executing a search warrant might be dangerous: The man lived in an isolated area, kept strange hours and was thought to have a secret bunker.

So they showed up at 3am on a Wednesday in December with a SWAT team.

When the man opened his front door, he greeted the deputies armed with a shotgun.

Though no shots were fired, deputies found an underground repository hidden under a trap door in the shed.

FBI agents executing a similar warrant on Tuesday (Feb 2) in a sprawling apartment complex in Broward county, nearly 970km south, faced a much more violent scenario.

A suspect that officials identified on Wednesday as David L. Huber, 55, who was under investigation in a case involving violent crimes against children, fatally shot two FBI agents, the bureau said.

"You can be prepared and equipped and do everything right - and things can go horribly wrong," Sheriff Michael Adkinson Jr of Walton county said.

Tuesday's fatal shooting has left the nation's tight-knit law enforcement community reeling as officials continue to examine evidence on the scene to understand how the FBI's case in Broward county turned into a deadly gunfight.

The shooting broke out before dawn when a group of FBI agents was executing a search warrant at Huber's apartment in Sunrise, Florida, north-west of Fort Lauderdale.

The suspect opened fire on members of the search team, federal officials said, and Huber was later found dead inside the apartment in an apparent suicide.

The shooting killed Special Agents Daniel Alfin and Laura Schwartzenberger and injured three other agents, all of whom had been released from the hospital as at Wednesday evening.

The Water Terrace apartment complex where the shooting took place was still teeming with law enforcement officers on Wednesday.

"To have somebody here the way this person was makes it very difficult, because you don't know who your neighbour is," said Mr Jorge Castillo, 76, who has lived at the complex for five years.

The shooting, which rivalled the worst in FBI history, returned the spotlight to the global scourge of online sex abuse - and to the dangers that law enforcement officers face when doing their frequent work of showing up at the doors of people under investigation to conduct searches or arrest them.

Florida has seen a spate of high-profile arrests involving online child exploitation recently, including a paediatrician in Broward county, a fourth-grade teacher in Boca Raton and a pastor in Central Florida.

Fourteen men were arrested in Manatee county in January. A December case resulted in the arrests of 13 men in Polk county.

Detective Michael Joo of the Broward County Sheriff's Office, who is assigned to the South Florida Internet Crimes Against Children Task Force, said in a recent podcast that in 2019, the unit initiated more than 200 cases and made more than 40 arrests for offences related to child exploitation.

"When people sit there and try and say: It's somewhere else, it's not here, maybe it's in California - we have those facts and figures that show that it's here in Broward county," he said.

In 2020, he estimates that cases have likely doubled because of children being online more during the coronavirus pandemic and vulnerable to solicitation from online predators.

Special Agent Debbie Garner, the commander of the child exploitation and computer crimes unit in the Georgia Bureau of Investigation, said her unit executed more in-person search warrants last year - about two a week - than any other unit in the bureau, a reflection of how pervasive online child exploitation cases have become.

"That's honestly a massive amount," she said.

In about a third of the cases, agents find that the suspect has a weapon, she said, "a sobering reminder that it could have been used".

When she started working with the unit seven years ago, it received about 180 "cyber tips" a month about possible online child predators.

The agents thought they were swamped. Now, they get about 1,000 tips a month, while funding for the unit has remained relatively flat.

"My agents will say that the worst thing is they have information that may help a child, and they know they may never get to it," she said.

"It's the overwhelming caseload that is the worst thing to them, and that they feel as though they will never reach all the children that need help."

Ms Carly Yoost, the founder and chief executive of the Child Rescue Coalition, a non-profit group based in Boca Raton, Florida, said that the state had been proactive about investigating online child abuse, as made clear by the recent arrests.

The police said the pastor arrested in Central Florida was found with abusive imagery on his computer; officers located him using the Child Rescue Coalition's technology, which monitors file-sharing services and some chat apps and areas of the Internet that are not accessible using a Web browser.

Ms Yoost said Florida did not stand out, however, in the number of reports the Child Rescue Coalition's technology locates.

Data provided to The New York Times by the organisation shows case numbers largely correlating with the population in most states.

Experts said it was uncommon to see suspects attack law enforcement during child sexual exploitation arrests; police officers are usually more worried about the suspects killing themselves or destroying evidence.

Still, investigators said they often found weapons on the property, meaning the threat of violence was always present.

"Every time my team approaches the house, I hold my breath, because you just don't know," said Sgt Jeff Swanson, the commander of an Internet crimes against children task force in Kansas.

Mr Benjamin Greenberg, a former US attorney for the Southern District of Florida, said online child exploitation cases were more common than people realised.

His office had a dedicated team of prosecutors who specialised in those cases, even though the Miami area is known mostly for other federal crimes, such as Medicare fraud.

"I don't think it's like healthcare fraud, where - for better or worse - we are the capital," he said.

With child pornography cases, he noted, online perpetrators could be anywhere: "Start to finish, you can engage in the criminal conduct without ever leaving your bedroom or your house."

That is why physical evidence, such as computers obtained with the help of a search warrant, is so important while putting together cases, and perhaps why agents descended on Huber's first-floor apartment on Tuesday.

Huber had worked as recently as April 2020 as a temporary technology contractor for Florida Blue, an affiliate insurance company of Blue Cross and Blue Shield.

He had previously worked as a temporary systems engineer with Modis, an employment agency, according to a financial statement he submitted in court when his wife filed for divorce in March 2016, in which he said he made US$80,000 (S$106,800) in 2015.

He indicated at the time that he was living at the Water Terrace apartments, the complex in Sunrise where the shooting took place.

Huber's former wife said in the court filings that the couple had married in 2000 and separated in 2009, and that their two boys had lived in nearby Pembroke Pines since they were born.

Ms April Evans, who lived in the Water Terrace complex until 2019, said she remembered the odd, grumbling man who lived in Apartment 102, Huber's listed address.

The unit was one flight down from her apartment, and while she did not know Huber's name, she recalled a frightening incident in which she learnt that the man in that unit had a gun.

An exterminator entered Ms Evans' apartment one day in 2018 or 2019 and said a man in the apartment downstairs had pointed a gun at him when he entered the unit, she said.

The exterminator appeared upset and told her that the man appeared not to have seen the notices plastered around the building alerting residents that an exterminator would be coming, Ms Evans said.

"I remember him saying, 'When I walked in, he had a gun in my face,'" she said. "He just looked shaken."

She said she had mostly avoided contact with her downstairs neighbour.

Their only interactions were when the two would pass by each other, she said, and he would only grumble in response to greetings from her and other neighbours.

She had noticed through his sliding glass door on one occasion that he seemed to have barely any furniture in his bedroom.

He lived alone but occasionally had two younger boys with him, she said.

Huber and his former wife had agreed in the divorce that their children would spend most of their time with their mother, visiting Huber on some Fridays and Saturdays.

In 2004, when the couple was still together, they registered a company called Computer Troubleshooters 0512 with the state.

Huber, who graduated from Broward College, received a commercial pilot's licence in 1994 and was also authorised to be a flight instructor, according to records from the Federal Aviation Administration.

A judge approved the couple's divorce in April 2016, finding that "irreconcilable differences exist and have caused the irretrievable breakdown of the marriage".

Huber's former wife lives in a gated community of two-storey houses with Spanish-tiled roofs and palm tree-lined streets in Pembroke Pines.

She could not be reached for comment on Tuesday or Wednesday.

A police cruiser was parked near one of the community's entrances.