Elon Musk said his planned acquisition of Twitter was “temporarily on hold” because of concerns about fake accounts, a surprise twist that jolted investors and raised questions about his willingness to go through with the $44 billion transaction.

Mr. Musk’s grenade came in a tweet posted at 5:44 a.m. Eastern Time that was followed just over two hours later by another saying he was “still committed to acquisition.” Lawyers close to Mr. Musk urged him to send that follow-up tweet, according to people familiar with the matter.