President Trump earlier this year purchased more than $1 million in the stock of Dell Technologies, a transaction that is drawing scrutiny after the Pentagon this week announced a $9.7 billion contract with the Texas-based computer company.

The sequence of events illustrates the complications that emerge when a sitting president continues, even if through financial advisers, to buy and sell publicly traded stocks like Dell.

In the first three months of this year, Mr. Trump’s investment portfolio executed more than 3,600 trades, a mandatory disclosure form shows, buying and selling shares in big banks, manufacturers and tech giants, among other companies.

The document shows a Dell stock purchase of $1 million to $5 million in February, and smaller amounts in the months since. (The document includes ranges in some cases, rather than specific amounts.)

The Trump family has argued that the president does not personally control the trading, saying these are automated trades placed by outside brokerage firms.

“This structure was intentionally designed to maintain a clear separation between President Trump and the independent third-party investment managers overseeing the accounts and avoid even the appearance of any conflict of interest,” the Trump Organization said in a statement earlier this month.

But this Dell stock purchase draws new attention to the inherent problems precipitated by the Trump family’s widespread investments since Mr. Trump returned to the White House. They include the family’s move into military drone manufacturing, cryptocurrency, mining and the predictions market, even as Mr. Trump oversees policy and government purchase decisions for those same sectors.

While outside advisers handle his financial accounts, they are not in a traditional “blind trust.” Rather, the president can still be aware of the moves for his investments.

The White House did not immediately respond on Thursday to a request for comment.

Mr. Trump also went out of his way in February to praise Dell Technologies and its founder, Michael Dell, at an event in Georgia that took place nine days after Mr. Trump’s purchase of more than $1 million in Dell stock.

The Georgia event took place after Mr. Dell and his wife pledged in December to donate $6.25 billion to help kick off the so-called Trump accounts program that creates special investment savings accounts with tax benefits for American children.

At the event, Mr. Trump celebrated the donation by the Dell family and went on to endorse their computer products. “Go out and buy a Dell computer,” Mr. Trump said.

The Pentagon contract, announced on Wednesday, gives Dell Federal Systems, a subsidiary of Dell, the right to oversee purchases across the Defense Department for Microsoft software, services and licenses — a “blanket purchase agreement” that the government estimated is worth $9.69 billion.

As president, Mr. Trump is exempt from a conflict of interest law that prohibits federal employees from taking actions in their official roles that benefit their own financial interest. He also is not required, as other federal workers generally are, to sell off financial holdings that might create a conflict of interest.

Robert Weissman, co-president of Public Citizen, a nonprofit group that has criticized the Trump family’s continued business activity, said the Dell stock purchase provides further evidence of the conflicts that have emerged in Mr. Trump’s second term.

“It is impossible to know where personal profit making ends and policymaking starts with this president,” Mr. Weissman said.