Twice this week, East Coast residents have been stunned by a sudden boom that rattled their windows and homes and left them questioning what had caused the commotion.

The latest thundering sound came from a fireball piercing through the air over the Boston area on Saturday around 2 p.m., NASA said.

The agency said the fireball reached speeds of up to 75,000 miles per hour as it was sucked into Earth’s atmosphere. Forty miles above the Massachusetts border with New Hampshire, the meteor fragmented.

The energy released as the rock broke up was equivalent to about 300 tons of TNT, accounting for the loud booms, NASA said. The event was not part of a meteor shower.

Before an explanation emerged, residents in the greater Boston area and Rhode Island had taken to social media to entertain a range of potential causes, including a nuclear attack or even an alien invasion. The Boston Police Department had fielded calls from across the state and officers had been dispatched in the Brighton, Mass., area, about 10 miles from downtown Boston.

Since it was cloudy over Boston on Saturday, residents may not have had a clear view of the burning rock, said Carl W. Hergenrother, the executive director of the American Meteor Society. The fireball could have been around the size of a basketball, and appears to have landed in the ocean, he said.

The initial mystery of the sound was partly fueled by the timing.

On Thursday, about 1,000 miles south of Boston, residents in South Carolina were left puzzled after a rumbling noise startled them around 5:30 p.m.

Hundreds described hearing clattering from the sky. There were no reports of injuries or deaths. On social media, some likened the sound to that of a bomb. Others said their rooms had shaken.

The local authorities in Richland County, S.C., did not immediately provide an explanation. NASA later confirmed that the booms over the state had not been caused by a meteor or fireball, but did not rule out the possibility that they had been the result of falling space debris or satellites.

Reports of the fireball over the Northeast on Saturday circulated from Quebec, throughout New England and into Maryland, Mr. Hergenrother said.

The asteroid and the Earth had crossed each other’s path, he said. These types of events happen over the continental United States around a half-dozen times a year. But widespread smartphone and social media access may have made these occurrences easier to track, he said.

Generally, space rocks plummeting to Earth do not cause injuries or deaths, he said. And when they land, the rocks do not cloud with steam, as fallen meteors are occasionally depicted to do in media.

“It’s not like in the movies where it leaves a big, you know, steaming hole in the ground,” he said. “It’s just, you walk outside and there’s a black rock on the ground where there shouldn’t be, where it wasn’t one before.”