(CNN) Federal prosecutors in New York who are investigating Rudy Giuliani and his associates have deepened their focus on Ukraine's state-run oil-and-gas company, having interviewed its CEO, Andriy Kobolyev, and seeking in recent weeks to speak to a key US embassy staffer in Ukraine, according to Kobolyev's attorney and people familiar with the matter.

Prosecutors have interviewed Kobolyev, the head of Naftogaz, which stands at the center of an attempted scheme by Giuliani associates Lev Parnas and Igor Fruman to replace Kobolyev with someone who could be more favorable to their own business interests.

"The Department of Justice requested an interview with Andriy Kobolyev," his attorney, Lanny Breuer, said. "He agreed and has voluntarily talked with the government attorneys. At this time, it would not be appropriate to comment further."

Kobolyev has said he would be willing to talk to prosecutors about all three men. "Everything is connected," he told Time last month.

Prosecutors have also made contact recently with the US embassy official, Suriya Jayanti, a foreign service officer based in Kiev. Jayanti handles Naftogaz issues at the embassy and has communicated with the company's management, according to an American energy consultant who operates in Ukraine, Dale Perry.

Prosecutors' interest in the activity surrounding Naftogaz indicates a possible expansion of their case against Parnas and Fruman, who have been indicted for an alleged scheme to funnel foreign money into US elections, and demonstrates they are looking closely at possible crimes related to foreign bribery. And it comes as prosecutors probe Giuliani's business interests.

As Parnas and Fruman pursued replacing the management of Naftogaz, CNN has reported they were working with Giuliani, President Donald Trump's personal attorney, to encourage Ukrainian officials to investigate Trump's political rival, Democratic presidential candidate Joe Biden and his son, Hunter Biden. Parnas and Fruman were also actively pushing to have US ambassador to Ukraine Marie Yovanovitch removed. There's no evidence of wrongdoing by either Biden.

Fiona Hill, Trump's former top Russia adviser, testified as part of the House impeachment inquiry that an American member of Naftogaz's board told her in May that a number of Ukrainians had complained to him about Giuliani discussing investigations and pushing to change the board of Naftogaz.

Jayanti couldn't be reached for comment.

A spokesman for the Manhattan US Attorney's office, which is prosecuting Parnas and Fruman for an alleged scheme to funnel foreign money into US elections, didn't comment. Parnas and Fruman have pleaded not guilty to those charges.

A lawyer for Giuliani has said he had no financial interest in Parnas and Fruman's liquified natural gas business. Giuliani has not been charged with any crime.

There has been no indication of wrongdoing by Naftogaz. A spokeswoman for the company declined to comment Wednesday. A spokesperson previously said Naftogaz has met with potential suppliers of liquified natural gas, and as part of that effort met with Parnas and Fruman.

Naftogaz and Kobolyev are part of a significant line of inquiry by federal prosecutors concerning Parnas and Fruman's activities. In recent weeks, prosecutors have also interviewed Andrew Favorov, a senior Naftogaz executive whom Parnas and Fruman attempted to recruit to become CEO, according to Favorov.

And they have sought to arrange interviews with at least one other individual with knowledge of Parnas and Fruman's approach to the company, according to people familiar with the matter.

In March, Parnas and Fruman approached Favorov, the senior Naftogaz executive, at an energy conference in Houston and asked if he would go along with their plan to oust Kobolyev and become its head, according to Perry, the Ukraine-based American energy executive and Favorov's former business partner.

"(Parnas and Fruman) basically just flat out said to him, hey, to do the deals we want to do, we were not able to get through to your CEO, and we think that the business needs a new CEO," Perry previously told CNN. Parnas and Fruman also told Favorov that Trump would soon replace Yovanovitch with an ambassador more amenable to their energy-business interests, according to Perry.

Perry said that he told Jayanti in March about his concerns about a possible threat to the integrity of Naftogaz's management and efforts by others that he perceived as potentially corrupt. A person familiar with the matter said Favorov also told Jayanti about his concerns regarding Parnas and Fruman.

Jayanti's name arose during the House of Representatives' impeachment inquiry, when she was identified as among those who overheard a July 26 phone call between Trump and US Ambassador to the European Union Gordon Sondland. She had been listed to appear for a private deposition in October with the committees looking into impeachment, but the deposition never occurred