NEW YORK  - Investigators have identified and arrested a suspect in the Boston Marathon bombings that killed three and injured 175, CNN reported Wednesday, citing official sources.

The suspect, described as a ‘dark-skinned male’, was picked up on security camera video from a department store along the marathon route, very close to the second explosion, sources said. Additional video from a Boston TV station was also helpful, the report said.

The station later issued an exclusive report saying officials had arrested the suspect.

Officials have scheduled a news conference for later Wednesday.

Earlier, investigators found a lid to a pressure cooker police said they think was used for a bomb, 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, neighbour 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.

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-Qaeda 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.”

US Secretary of State John Kerry, who formerly represented Massachusetts in the US 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.”

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.

Meanwhile, a letter addressed to US President Barack Obama at the White House contained a suspicious substance and is being tested, the Secret Service said on Wednesday.

The letter arrived at the White House mail facility on Tuesday, the same day a letter addressed to Senator Roger Wicker, a Republican, tested positive for the poison ricin.

The Wall Street Journal said officials indicated the letter to Obama initially tested positive for ricin.

CNN said authorities evacuated parts of the US Capitol complex on Wednesday because of suspicious packages found on the first and third floors of the Hart Senate Office Building and a suspicious envelope at the office of Senator Richard Shelby, also a Republican, in the Russell Senate Office Building.

The Hill said staffers in Democratic Senator Carl Levin’s office reported a suspicious letter to law-enforcement and the building was evacuated as a precaution.

Security had been increased in House and Senate office buildings since Tuesday.

Capitol Hill police reportedly were questioning a man with a backpack in the Hart Office Building, CNN said.

The Hill said the letter addressed to the President was intercepted and sent to a Secret Service mail screening facility off-site from the White House for testing. The Secret Service has not said whether the letter tested positive for the poison.

“This facility routinely identifies letters and parcels that require secondary screening or scientific testing before delivery,” the Secret Service said in a statement. “The Secret Service White House mail screening facility is a remote facility, not located near the White House complex, that all White House mail goes through.” Media reports said the White House letter — like the letter sent to Wicker — was postmarked Memphis, Tennessee, and contained similar language.