Businesses and TV personalities have offered more than $300,000 (£224,000) to catch those responsible for a camper van blast in the US city of Nashville.
The explosion rocked the city early on Christmas Day, injuring three people and knocking out communications systems across the state of Tennessee.
Police believe the powerful blast was caused deliberately.
Law enforcement agents have searched the home of a possible person of interest in nearby Antioch.
No motive has yet been established for the explosion, and no-one has yet said they were behind it. Possible human remains have been found near the blast site.
FBI Special Agent in Charge Douglas Korneski said officials had received about 500 tips and that they were looking at a "number of individuals" in possible connection to the explosion.
Officers from the Bureau of Alcohol, Tobacco, Firearms and Explosives are also involved in the investigation.
Earlier, Tennessee Governor Bill Lee said he toured the site, saying on Twitter it was a "miracle" that no-one had been killed. He said he had asked President Donald Trump for a federal emergency declaration for his state to aid relief efforts.
Businessman Marcus Lemonis is the latest to donate to a reward pot, pledging $250,000.
"We can't have our streets terrorised like this," tweeted Mr Lemonis, who hosts reality TV show The Profit.
. @MNPDNashville @JohnCooper4Nash I would like to put up a $250,000 reward to anyone who provides information that leads to the arrest and conviction inside of your process, of the this Nashville incident. We can’t have our streets terrorized like this. #horror pic.twitter.com/k9fNjRjklZ
— Marcus Lemonis (@marcuslemonis) December 25, 2020
Reward pledges began on Friday after a local tourism body, Nashville Convention & Visitors Corp, made an initial contribution of $10,000, later increasing it to $35,000.
What happened in Nashville?
Officers responded to reports of gunshots just before 06:00 local time (12:00 GMT) on Christmas Day in an area of the city known for its restaurants and nightlife.
Shortly afterwards, they found a camper van broadcasting a warning message to leave the area.
The van exploded a few minutes later, the force of the blast knocking an officer off their feet, police said. It is still unclear if anyone was inside the camper van at the time.
Police have released this image of the van - described by Nashville police as a recreational vehicle (RV) - arriving at the scene early on Friday.
BREAKING: This is the RV that exploded on 2nd Ave N this morning. It arrived on 2nd Ave at 1:22 a.m. Have you seen this vehicle in our area or do you have information about it? Please contact us via Crime Stoppers at 615-742-7463 or online via https://t.co/dVGS7o0m4v. @ATFHQ pic.twitter.com/JNx9sDinAH
— Metro Nashville PD (@MNPDNashville) December 25, 2020
The van blew up outside a building belonging to the telecoms giant AT&T, which also occupies an office tower nearby.
Buildings suffered structural damage, windows were blown out, and trees were felled. Videos posted on social media showed water from damaged pipes running down walls as alarms howled in the background.
Police emergency systems were knocked out across the surrounding state of Tennessee.
Telephone, internet and fibre optic TV services were also disrupted in Tennessee, Mississippi, Kentucky, Alabama and Georgia, according to telecoms firm AT&T.
Resident Buck McCoy said he had been woken up by the blast. He posted a video on Facebook, showing some of the damage done, with alarms howling in the background.
"All my windows, every single one of them got blown into the next room. If I had been standing there, it would have been horrible," Mr McCoy told AP. "It felt like a bomb. It was that big."