"Contrary to widespread reporting, no arrest has been made in connection with the Boston Marathon attack," the FBI said on its website. "Over the past day and a half, there have been a number of press reports based on information from unofficial sources that has been inaccurate. Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting."
Also Wednesday, the White House said President Obama and the first lady will travel to Boston Thursday to attend "Healing Our City: An Interfaith Service" dedicated to those who were gravely wounded or killed in Monday's bombing near the finish line of the Boston Marathon.
Meanwhile, security officials ordered an evacuation at the Moakley Federal Courthouse in Boston, but did not give details. Offices are routinely evacuated after telephoned bomb threats.
Before the FBI knocked down the reports, CNN and other news outlets, citing sources they did not name, had been reporting investigators have identified and arrested a suspect in the bombings that killed three and injured 175.
The sources said the suspect, described as a "dark-skinned male," was picked up on security camera video from the Lord & Taylor department store along the marathon route, very near the second explosion.
Earlier, investigators found a lid to a pressure cooker police said they think was used for one of the bombs, CNN reported.
Citing a federal law enforcement source it described as having firsthand knowledge of the investigation, CNN said investigators found the item on the roof of a building near where the bombs went off.
The FBI was seeking leads into who may have been responsible for the attack, combing through photos and videos taken at the race's finish line shortly before two bombs exploded at mid-afternoon Monday.
FBI Special Agent in Charge Richard DesLauriers has appealed to the public for information.
"The person who did this is someone's friend, neighbor or relative," DesLauriers said at a televised briefing.
"Someone knows who did this. The cooperation of the community will play a crucial role in the investigation," he said.
"We are asking anyone who may have heard someone speak about the marathon or the date of April 15 in any way that indicated he or she may target the event," DesLauriers said.
Investigators also are studying pieces that appear to have come from the explosive devices, fashioned from pressure cookers packed with ball bearings and nails.
CBS reported al-Qaida's online forums and chat rooms were rife with speculation on who was responsible. The terror network has been encouraging so-called lone wolf actions -- attacks by jihadists acting on their own to reduce the likelihood their plans would be discovered.
"The investigators are struggling with that question as al-Qaida, on the other side of the world, is wondering 'Was that us?'" said John Miller, a former assistant FBI director and a senior CBS correspondent.
Tom Fuentes, a former FBI assistant director told CNN, based on what is known so far about the attack, it "has the hallmarks of both domestic and international" attacks.
"If your experience and your expertise is Middle East terrorism, it has the hallmarks of al-Qaida or a Middle East group," he said. "If your experience is domestic groups and bombings that have occurred here, it has the hallmarks of a domestic terrorist like Eric Rudolph in the 1996 Atlanta Summer Olympics bombings."
U.S. Secretary of State John Kerry, who formerly represented Massachusetts in the U.S. Senate, said Wednesday Boston "is not going to be intimidated" by the attack.
"But we are going to find out who did this," he said during testimony before the House Foreign Affairs Committee on Capitol Hill, Wednesday. "The police work being done is extraordinary. The FBI is remarkable. There is a great deal of forensic evidence. We are hopeful we can bring people to justice."
No one has yet claimed responsibility for the blasts, although the Pakistani Taliban denied any connection.
Among the injured, more than 20 were in critical condition, hospitals said. Thirteen people had to have limbs amputated.
Officials said they have received more than 2,000 tips from the public.