PHILADELPHIA—A group of people dressed in black vandalized at least two federal buildings in downtown Philadelphia on New Year’s Eve, and police said they recovered Molotov cocktails and other suspicious devices as they arrested seven of them, authorities said Friday. Officers spotted about 50 people, all in black, around 8:30 p.m. Thursday at the historic U.S. Customs House, police said. And a 25-year-old man was seen throwing a brick through the windows of a federal building and courthouse shortly before 9 p.m., police said. He fled with a 24-year-old man and two 23-year-old women, all of whom were arrested. Damage to the building was estimated at about $3,000. Anti-police and anti-prison graffiti were scrawled on walls, a sheriff’s van was defaced and several windows were broken. Three other people were also arrested in the area, a 25-year-old man and two women ages 22 and 26. Police said the man was …