Investigators have spotted a Boston Marathon bombing suspect from security video taken before two blasts ripped through central Boston on Monday, a US law enforcement source has said, in what is potentially the biggest break in the case yet. No arrests had been made, and the suspect in the video had not been identified by name, two US government officials said on Wednesday. Police considered making an appeal to the public for more information at a news conference, a US government source said, but the FBI cancelled that news conference after delaying it several times, Boston police said. The bombings, as well as subsequent reports that someone tried to mail a suspected ricin letter to US President Barack Obama – the second report of such a letter in two days – have created a climate of uncertainty in the country. Earlier the FBI had said there was no indication of a connection between the ricin letters and the Boston bomb attacks. The federal courthouse in Boston was evacuated amid reports a suspect for Monday’s bombings at the Boston Marathon was due to appear in court, but officials said they cleared the area because of a bomb scare.