WASHINGTON (NYTIMES) - President Donald Trump's former national security adviser, Michael Flynn, is facing a mounting tangle of potential legal troubles arising from his business dealings with foreign entities and his interactions with both executive branch and congressional investigators.

One set of problems stems from Russia. In 2015, Russian-linked companies paid Flynn more than US$65,000, including about US$45,000 from the state-backed Russian television network RT for a December trip to Moscow, where he delivered a speech and sat next to President Vladimir Putin at a dinner.

In December, a US wiretap of the Russian ambassador to the United States, Sergey Kislyak, intercepted conversations he had with Flynn discussing the Obama administration's imposition of sanctions on Russia for meddling in the election.

The other set stems from Turkey. Last year, Flynn's lobbying firm, Flynn Intel Group, was paid more than US$500,000 by Inovo BV, a Dutch corporation. Inovo is owned by Ekim Alptekin, a Turkish-American businessman with ties to the administration of President Recep Tayyip Erdogan of Turkey.

The work centred on research to discredit Fethullah Gulen, a Muslim cleric who lives in Pennsylvania, and whose extradition Erdogan has been seeking.

Robert Kelner, a lawyer for Flynn, declined to comment. But in a letter to the Senate this week, Kelner and other lawyers portrayed Flynn as being treated unfairly, saying he was "the target on nearly a daily basis of outrageous allegations."

Here is a breakdown of some of the possible legal problems for Flynn.

Making False Statements

Investigators are scrutinising whether several of Flynn's statements violated a statute that makes it a felony, punishable by five years in prison, to make a false statement to the government. To convict, prosecutors must prove that the violation was knowing and willful, not merely inadvertent.

In January 2016, Flynn, a former three-star general who retired in 2014 after being forced out as head of the Defence Intelligence Agency, applied to have his security clearance extended. As part of the process, he filled out and signed a SF-86 form. It directed him to disclose his foreign travel, compensation and contacts.

It also referred to the false-statement statute as part of a certification that the information was truthful and complete.

After viewing the form, Rep. Elijah Cummings of Maryland, the ranking Democrat on the House Oversight Committee, said Flynn had concealed his Russia trip and the compensation when he filled it out.

In February 2016, investigators reviewing Flynn's application to renew his security clearance interviewed him. According to a letter this month from Cummings, documents suggest Flynn falsely stated in that interview that US companies had funded his Moscow visit and that he had only "insubstantial contact" with foreigners, despite the dinner with Putin.

Flynn's legal team has said he briefed the Defence Intelligence Agency about the trip. Attorney General Jeff Sessions and Jared Kushner, Trump's son-in-law and adviser, also failed to mention encounters with Russian officials on their SF-86 forms.

Separately, four days into the Trump administration, FBI agents interviewed Flynn about his conversations with Kislyak in December. Several news outlets have reported that he initially told the FBI that he had not discussed sanctions on Russia but then backtracked and said he could not remember.

Taking Foreign Payments Without Permission

The Constitution's emoluments clause prohibits people who hold government positions from receiving payments from foreign governments without prior consent from Congress. This rule extends to retired military personnel, and Congress enacted a statute to carry it out. It says retired military personnel may accept such compensation only if they get permission from their service secretary and the secretary of state.

Flynn apparently did not obtain permission before taking compensation from RT or Inovo. That failure is complicated by the fact that they are companies, not governments, but the US government has portrayed RT as an arm of the Kremlin.

Alptekin has told reporters he hired Flynn with his own funds and did not coordinate with the Turkish government. But congressional investigators have expressed skepticism after receiving classified briefings.

"As a former military officer, you simply cannot take money from Russia, Turkey or anybody else," Rep. Jason Chaffetz, R-Utah, chairman of the House Oversight Committee, said in April.

"And it appears as if he did take that money. It was inappropriate, and there are repercussions for the violation of law."

Such a violation would not constitute a criminal offence; a Pentagon ethics document says the penalty is forfeiting the funds through withheld retirement pay, not prison time. The acting inspector general of the Defence Department, Glenn Fine, told Congress that his office had begun investigating whether Flynn violated emoluments rules.

Failure to Register As a Foreign Agent

The Foreign Agents Registration Act, or FARA, requires that people representing foreign governments or political parties register with the Justice Department within 10 days. Under departmental regulations, this rule extends to lobbyists hired by foreign companies if the "principal beneficiary" of their work is a foreign government.

In September, after signing the contract with Inovo, the Flynn Intel Group registered as an ordinary lobbyist for that firm but did not make a FARA filing, which requires greater disclosure.

However, in March, after his ouster from the Trump administration, Flynn's lawyers filed a belated FARA registration about the arrangement "to eliminate any potential doubt."

"Because of the subject matter of Flynn Intel Group's work for Inovo BV, which focused on Mr Fethullah Gulen, whose extradition is sought by the government of Turkey, the engagement could be construed to have principally benefited the Republic of Turkey," the lawyers wrote.

Violating FARA is a felony punishable by up to five years in prison, although legal specialists say the Justice Department rarely prosecutes that charge.

Failure to Comply With Subpoenas

Several congressional committees have been seeking information about Flynn's activities, and there are signs more subpoenas are coming. The Senate Intelligence Committee voted unanimously Thursday to give its top Republican and Democrat broad authority to issue subpoenas.

The most advanced subpoena is one the Intelligence Committee issued to Flynn on May 10 seeking all records of his meetings and communications with Russian officials, and of his communications with Trump's campaign about Russia in the 18 months before Trump's inauguration.

On Monday, Flynn's lawyers told the committee that he was invoking his right against self-incrimination to avoid complying with the subpoena. Although invoking the Fifth Amendment is usually associated with live testimony, his lawyers argued that it fell under a precedent for document production requests that are so broad that the act of acknowledging whether any such files exist could amount to self-incriminating testimony.

The committee can vote to hold anyone refusing to comply with a subpoena in contempt of Congress. If the full House or Senate chamber votes likewise, the dispute moves into court.

The Justice Department would have to decide whether to prosecute the subpoena recipient for criminal contempt of Congress, which carries a penalty of one to 12 months in prison. Lawmakers could also file a lawsuit asking a federal judge to order compliance with their subpoena.