Israeli Prime Minister Benjamin Netanyahu attends a Likud party meeting at the Knesset, the Israeli parliament, in Jerusalem on Jan. 2. (Ronen Zvulun/Reuters)

Israeli police investigators questioned Prime Minister Benjamin Netanyahu for three hours Monday night as part of a criminal probe into whether he received bribes from wealthy businessmen.

Although no charges have been filed, the questioning at the prime minister’s official residence marked an escalation in a long-running graft investigation. Afterward, the Israeli police released a short statement confirming that they had questioned Netanyahu over the allegations, but they provided no further details.

The Israeli media, however, reported that the investigation centered on evidence that the prime minister had been given valuable gifts with the apparent expectation of political favors in return.

Netanyahu has vehemently denied wrongdoing, and did so again Monday.

“I have told you and I repeat: There will be nothing because there is nothing,” the prime minister told his ruling, right-wing Likud party.

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Addressing the opposition, he said, “You will continue to inflate hot air balloons, and we will continue to lead the state of Israel.”

Opposition leaders said they were not celebrating the prime minister’s legal struggles. “This isn’t a happy day. This is a hard day for the state of Israel,” center-left leader Isaac Herzog said, according to Israeli media accounts.

Netanyahu’s predecessor, Ehud Olmert, stepped down in 2009 over corruption allegations. He is serving a 19-month prison sentence for bribery and obstruction of justice.

Allegations have long swirled around Netanyahu, who has been in office for eight years during his second stint as prime minister. But nothing has ever stuck.

Police recommended in 2000 that charges be filed against Netanyahu and his wife, Sara, based on evidence that they had kept official gifts that should have been returned to the Israeli state. But the case was dropped because of insufficient evidence.

Large black screens were mounted around the prime minister’s residence in an upscale area of Jerusalem on Monday night in an apparent bid to shield from view the arrival and departure of investigators.

Several Israeli media outlets reported that Netanyahu is the subject of two police investigations. The questioning Monday night was apparently related to the less serious of the two, which concerns gifts from businessmen, according to the media reports. Little is known about the other inquiry.

Police have reportedly interviewed dozens of witnesses, including a prominent American backer of Netanyahu’s, businessman Ronald Lauder.

Netanyahu has had a stormy relationship with President Obama, culminating in recent weeks with angry exchanges over a U.N. resolution condemning Israel’s settlement expansion. But the prime minister appears to be on much better terms with President-elect Donald Trump, with the two men sharing warm words for one another on Twitter.

Ruth Eglash contributed to this report.