SUMMERVILLE — A massive branch about 5 feet in diameter snapped off the Middleton Oak as Sidney Frazier worked elsewhere at Middleton Place.

"I was on the other end of the garden, and I could hear it," said Frazier, Middleton's vice president of horticulture. "It sounded like thunder."

The first branch broke off around 2 p.m. Wednesday. Later that night, a far larger limb cracked and slammed to the ground, blocking a garden path.

Robert Van Pelt, a research ecologist with the University of Washington's College of Forest Resources, knows the Middleton Oak better than most.

In 2004, he and four colleagues spent 10 hours in the tree, one of the nation's largest, meticulously measuring its limbs.

"Everyone goes to see the Angel Oak (on Johns Island), but I was more impressed with that tree," Van Pelt said. "It is a lot bigger. The other has a wide crown and all that, but Middleton is actually the largest oak in all of eastern North America. It might be the largest oak in the United States."

The tree began growing on the Ashley River's banks centuries before 1741, when Henry Middleton — later president of the First Continental Congress — started a plantation there.

The tree eventually became an iconic feature of a 60-acre landscaped garden that has no shortage of iconic views.

"It's one of our most popular wedding spots," said Pat Kennedy, Middleton's vice president of marketing. "A lot of people have been engaged and married under this tree."

The limbs that fell were connected with steel cables, but Frazier said those lines were designed not to support the branches' weight but simply to prevent them from swaying too much in a strong wind.

Van Pelt said Middleton is the state's largest tree if you measure the volume of its wood, which was almost 5,000 square feet before the loss of two of its limbs. "Angel Oak would probably be around 3,500 square feet or something, though they're both amazing."

The only larger tree on the East Coast is a bald cypress in Florida, he said.

On Thursday afternoon, a football-field size section of the gardens was cordoned off with yellow caution tape so visitors wouldn't get too close.

Frazier looked on as five workers with Bartlett Tree Experts propped two telephone poles underneath branches closest to the largest one that snapped. Other Bartlett experts plan to fly to Charleston today to examine what happened and help advise what to do next.

At this point, there are more questions than answers.

No one is sure why the limbs broke, when more might break, what can be done to stabilize the tree or when Middleton visitors might be allowed to walk underneath it again.

"Whatever we do, we've got to make sure it's the right thing the first time," Frazier said. "We won't get a second chance."