This story has been updated

The Department of Justice is planning to bring criminal corruption charges against Sen. Robert Menendez (D-N.J.), a U.S. official confirmed Friday, casting renewed attention on the question of whether the senator used his powers to improperly benefit a close ally and political donor.

The charges, which the Justice Department could unveil within weeks, are expected to stem from an investigation involving Menendez and Salomon Melgen, a Florida-based ophthalmologist who has donated to Menendez and other Democrats and is a close friend of the senator's.

At a brief appearance where he took no questions Friday night, Menendez said, "I have always conducted myself appropriately and in accordance with the law."  He later added: "I am not going anywhere."

“As we have said before, we believe all of the Senator's actions have been appropriate and lawful and the facts will ultimately confirm that," said Menendez spokeswoman Tricia Enright. "Any actions taken by Senator Menendez or his office have been to appropriately address public policy issues and not for any other reason.

CNN first reported the planned charges against Menendez. The White House declined to comment on the report. Approached by a Wall Street Journal reporter about the matter, Attorney General Eric H. Holder Jr. responded: "I can't comment on that."

Menendez's relationship with Melgen has been under scrutiny for more than two years. Concerns from the FBI and federal prosecutors about the statute of limitations running out on possible charges have spurred the Justice Department to move ahead swiftly, law enforcement officials said.

[Past coverage: For Robert Menendez, a senator set apart, closeness to rich donor draws scrutiny]

During the course of the investigation, grand juries had convened in both South Florida and North New Jersey. But many legal experts in white collar defense had raised doubts about federal prosecutors’ ability to pursue charges against the senator, and suspected the case might have died.

Menendez’s actions had raised questions because he appeared to use his official position and even his powerful status as chairman of the Senate Foreign Relations Committee to go to bat to help a donor’s bottom line – both with the State Department and with the Centers for Medicare and Medicaid Services. But that donor was unusual; he was a businessman but also a close and longtime friend of the senator’s. Prosecuting officials for helping people they could prove were genuine personal friends has traditionally been considered an uphill battle.

Twice in recent years – in 2009 and 2012 – Menendez and his top staff spoke directly to top federal health agency officials about their finding that Melgen had overbilled the government by $8.9 million for care at his clinic.

Menendez repeatedly questioned whether federal auditors had been fair in their assessment of Melgen’s billing for eye injections to treat macular degeneration. His office said he questioned the fairness and consistency of the federal agency in its decision-making with all doctors, not just Melgen.

But Melgen was a man whose personal life also drew scrutiny. He was seen at times with dates while entertaining Menendez near his vacation home in the Dominican Republic.

Another area in which Menendez appeared to use his position to help the doctor came after Melgen became the chief investor in a company holding a long-dormant port security contract in the Dominican Republic. The contract called for paying lucrative fees for security screening of ships coming into the port.

In the summer of 2012, as Melgen donated $700,000 to support the senator and other Democrats, Menendez pressed for the United States to push the Dominican Republic to put the contract into effect. In a July Senate hearing he scheduled on Latin American businesses, Menendez urged officials from the Commerce and State departments to apply pressure to countries that didn’t honor agreements with U.S. businesses. Without naming Melgen, Menendez highlighted the contract to provide security in the Dominican port.

In recent weeks, law enforcement sources said the FBI was pushing prosecutors to charge in the case, and that the top prosecutor in New Jersey, where a grand jury has been hearing evidence, supported the case.

[READ: Federal court documents related to investigation of Sen. Robert Menendez]

On Capitol Hill, Menendez has maintained a prominent role among Senate Democrats in recent months, even after losing his gavel as chairman of the Foreign Relations Committee as Republicans assumed control of the Senate.

The son of Cuban immigrants, he was a lonely but powerful voice among Democrats criticizing President Obama's decision in December to normalize relations with Cuba. He decried the trade of convicted Cuban spies for an imprisoned American aid worker, saying it "vindicated the brutal behavior of the Cuban government."

In the new year, Menendez has assumed a leading role in Senate debates over America's relations with Iran -- again taking a harder line on the issue than the Obama administration.

He is a co-author of two bills with broad Republican support, one that would strengthen economic sanctions against Iran, another that would give Congress the ability to review any deal negotiated in multiparty talks over Iran's nuclear program now underway in Switzerland.

Menendez withdrew his support for the latter bill this week after Republican leaders moved to bring the measure to the Senate floor before the negotiations expire later this month. But he made clear Tuesday -- the same day Israeli Prime Minister Benjamin Netanyahu addressed Congress in a speech Menendez lauded -- that he still supported a congressional review of any deal that comes out of the talks.

The latest Monmouth University poll, from February, suggested 48 percent of New Jersey residents approved of the job Menendez was doing while 26 percent disapproved.

Menendez was first appointed to the Senate in 2006, serving in the House before that. He was elected to a full term later that year and easily won reelection in 2012.

If Menendez were to leave office, his departure would shake up New Jersey politics. Most ambitious New Jersey Democrats have been focused on running to succeed Gov. Chris Christie (R) in 2017, and not on Menendez's seat. Philip Murphy, a former Goldman Sachs official who served as President Obama's ambassador to Germany, has said he's considering a gubernatorial run, as has Jersey City Mayor Steven Fulop.

The state's congressional delegation is dominated by Democrats, but most of them are much older, including 78-year-old Rep. Bill Pascrell (D-N.J.) and 64-year-old Albio Sires (D-N.J.). Rep. Frank Pallone (D-N.J.) has expressed interest in higher office in the past but just won a grueling battle to serve as the top Democrat on the House Energy and Commerce Committee.

Mike DeBonis, Scott Clement, David Nakamura and Ed O'Keefe contributed to this story.