Donald Trump Jr. said Tuesday that he agreed to a meeting with a Russian lawyer during last year’s campaign because he believed that the individual had information about Democratic candidate Hillary Clinton.

“I thought [the information] was Political Opposition Research,” Trump Jr. wrote on Twitter, in a statement explaining why he’d agreed to meet the lawyer, Natalia Veselnitskaya.

At the same time, Trump Jr. released emails showing that he was told Veselnitskaya was a “Russian government attorney” whose information about Clinton was part of “Russia and its government’s support” for his father.

On Tuesday, ethics lawyers and veterans of past Washington campaigns said this sort of encounter — a campaign’s core officials meeting with a foreigner, to talk about dirt dug up by a foreign government — is highly unusual, even in the take-no-prisoners world of opposition research.

In most campaigns, what’s referred to as “opposition research” is usually more mundane: Paid researchers comb through public records and legislative histories, looking for information that makes a rival candidate look bad.

(Elyse Samuels,Jenny Starrs/The Washington Post)

The intent is usually to produce a report that can be handed to reporters, along with documentation that would stand up to a journalist’s scrutiny. Campaign lawyers said it was very uncommon to have a foreign state offer to provide incriminating information.

Could the meeting also have been against the law? Legal experts said that depends on whether the aid promised by Veselnitskaya could be counted as a “thing of value” for legal purposes and whether Trump Jr. could be said to have “solicited” it by agreeing to the meeting.

It is illegal, under U.S. law, for a campaign to solicit or accept any contributions from foreign nationals or foreign governments.

Election lawyers and campaign veterans said they would have advised the younger Trump to avoid a meeting with someone identified as an emissary of the Russian state.

“He knew” that the lawyer’s information came from the Russian government, said Larry Noble, of the nonpartisan Campaign Legal Center. “This is clear. In this case, any question we had about what he knew has been removed. Because the email says the Russian government is helping.”

Even those who doubted that the meeting posed any legal problems called it unusual.

Gary Maloney, a longtime GOP opposition researcher, said he was surprised that Trump Jr. attended the meeting in person, along with the president’s son-in-law, Jared Kushner, and Paul Manafort.

Maloney said he would have expected a consultant or a researcher to go instead. That way, if the meeting caused political blowback, it would not be connected to the candidate’s family or top campaign officials.

“I do not fault Donald Jr. for wanting to win. He seeks more data, more leads — nothing wrong there,” Maloney wrote in an email. “But where was the campaign team? The manager’s job is to protect the candidate and family from blowback. I worked in Bush ’88 — would [campaign manager] Lee Atwater have allowed George W. at such a sitdown? Inconceivable. Would John Podesta have allowed Chelsea Clinton in this meeting? Impossible.”

The emails released by the younger Trump show that he was approached by a music publicist, Rob Goldstone, who represented a Russian pop star, Emin Agalarov. The pop star’s father, Aras Agalarov, is a Russian real estate developer close to Russian President Vladi­mir Putin.

The Agalarovs and President Trump had met through previous business dealings, and they were close enough that Trump made a cameo in one of Emin Agalarov’s music videos, filmed when Trump visited Moscow in 2013.

Goldstone had written to invite the younger Trump to meet with Veselnitskaya, who he said had damaging information about Clinton.

“This is obviously very high level and sensitive information but is part of Russia and its government’s support for Mr. Trump — helped along by Aras and Emin,” Goldstone wrote.

Trump Jr. responded enthusiastically. “If it’s what you say I love it especially later in the summer,” he wrote.

Both the younger Trump and his attorney have portrayed this meeting as a brief, unproductive and ultimately unimportant episode in a long, busy campaign. “The bottom line is that Don, Jr. did nothing wrong,” Alan Futerfas, Trump Jr.’s attorney, said in a statement Monday night, before the emails were released.

President Trump’s lawyer has said the president was not aware of the meeting and did not attend. On Tuesday, the only response from Trump himself was a statement issued through White House spokeswoman Sarah Huckabee Sanders: He called his son a “high-quality person” and praised him for his transparency.

On Monday, even before the emails were released, the watchdog group Common Cause filed a complaint with the Justice Department and Special Counsel Robert S. Mueller III, asking Mueller to investigate whether the younger Trump had violated election law.

The complaint says there is “reason to believe that Donald J. Trump for President, Inc. and Donald Trump Jr. violated [a federal ban] on soliciting a contribution from a foreign national.”

Paul Ryan, a Common Cause lawyer, said this argument was based on the law’s definition of “contribution” as “anything of value . . . for the purpose of influencing any election for federal office.”

He said Tuesday that the emails only strengthened this case, since they showed that Trump Jr. was explicitly told that the information he would get came from the Russian government.

Campaign-law attorneys said what Trump Jr. was offered might be considered a “thing of value,” if the information he was seeking had cost someone money to produce — or if it was something that a campaign might have paid for.

In this case, what Goldstone offered was, in his words, “some official documents and information that would incriminate Hillary and her dealings with Russia.”

Rick Hasen, a law professor at the University of California at Irvine, wrote on his Election Law Blog that this might fit the definition of “anything of value.” He pointed to a 1990 Federal Election Commission advisory opinion that said polling data — provided to a campaign by a volunteer — could be counted as a thing of value.

In this case, Trump Jr. said he received nothing valuable, arguing that the lawyer “had no information to provide” and instead steered the conversation to a U.S. law that has imposed sanctions on some Russian officials because of human rights violations.

But the unusual nature of the meeting led many campaign veterans to recall how a similarly fraught situation was handled during the 2000 presidential campaign.

That September, as then-Vice President Al Gore, the Democratic nominee, prepared for a debate, his staff received a package with materials stolen from Republican George W. Bush’s campaign. Tom Downey, a former congressman and Gore adviser, recalled an internal debate about what to do.

They decided to call the FBI.

“I’ll never forget the agent coming over here and saying to me, ‘Congressman, what’s the crime?’ I said to him: ‘Well, right now this is probably worth a small fortune to somebody. . . . This is somebody sending illegal property to me,’ ” Downey said in a phone interview Tuesday.

He said some Gore supporters were unhappy with that decision, which forfeited a possible advantage in the debate.

“All I could see was myself before some panel of senators,” he recalled, “with my hand raised in the air, [being asked,] ‘And why didn’t you turn it over to the FBI?’ ”