Revelations that Israel was spying on closed-door US-led negotiations between the five permanent members of the UN Security Council and Iran, and then selectively leaking intelligence with the aim of scuttling any nuclear deal, is only the latest saga in a tortured history of the close alliance between the US and its best friend in the Middle East.

First reported by the Wall Street Journal, Israeli spies eavesdropped and acquired confidential information about US briefings on Iran from “informants” and “diplomatic contacts” in Europe.

Israel denied the allegations, saying any intel they obtained was through surveillance of Iranian leaders.

Most shocking–and upsetting to administration officials, however–was Israel’s sharing of intelligence it collected with Members of Congress in order to drum up opposition to a potential nuclear agreement.

“It is one thing for the US and Israel to spy on each other. It is another thing for Israel to steal US secrets and play them back to US legislators to undermine US diplomacy,” an unnamed senior US official told the WSJ.

Lawmakers have reacted to the allegations with some concern as well. On Tuesday, Sen. Angus King (I-Maine) said that the reports, if true, are “somewhat disturbing.”

Speaker of the House John Boehner–who kicked off the latest diplomatic dispute between the US and Israel by inviting Prime Minister Benjamin Netanyahu to address Congress earlier this month without first clearing it with the White House–reacted skeptically, but with hints of alarm, to the reports of spying.

“I was shocked by the fact that there were reports in this press article that information was being passed on from the Israelis to members of Congress. I’m not aware of that at all,” he claimed on Tuesday.

President Obama, meanwhile, declined on Tuesday to make any sort of direct remark on the matter. He stressed that his administration has briefed both Congress and the Israeli government on the discussions, and said that any final deal will be subject to scrutiny.

“As a general rule, I don’t comment on intelligence matters in a big room full of reporters. And I think I’ll continue that tradition,” he remarked, during a joint press conference with Afghan President Ashraf Ghani.

But Israel’s employ of its intelligence services to abuse its alliance with the US is well-documented, particularly in troves of internal intelligence records released in 2013 by former NSA contractor Edward Snowden.

In one leaked document, an NSA official noted that “one of NSA’s biggest threats is actually from friendly intelligence services, like Israel.”

A US National Intelligence Estimate released by Snowden ranked Israel, behind China and Russia, as the “third most aggressive intelligence service against the U.S.”

Another document listed Israel as a routine offender of “espionage/intelligence collection operations and manipulation/influence operations…against U.S. government, military, science & technology and Intelligence Community.”

Despite the numerous reported instances of exploitation, the Snowden cache showed that the NSA continued over the past few years to share enormous amounts of data with Israeli intelligence services.

In September 2013, the Guardian reported how American spies have shared “raw intelligence data with Israel without first sifting it to remove information about US citizens.” The agreement included no restriction on how Israelis are to use the data—a trove that includes information belonging to American citizens.

Spies reviewing the history of the US-Israeli intelligence partnership noted in a separate secret document released by Snowden that “Balancing the [intelligence] exchange equally between US and Israeli needs has been a constant challenge. In the last decade, it arguably tilted heavily in favor of Israeli security concerns.”

The report went on to reveal that US intelligence officials believe Israel represents the “NSA’s only true Third Party [counter-terrorism] relationship being driven almost totally by the needs of the partner.”