Much ink has been spilled over the last 50+ years on two Cold War spy cases: Rosenberg and Hiss. Here is a new piece of "evidence" (hearsay, technically, since it is an out-of-court statement) in the Rosenberg case.

NY Times
Figure in Rosenberg Case Admits to Soviet Spying
By SAM ROBERTS
Published: September 11, 2008
In 1951, Morton Sobell was tried and convicted with Julius and Ethel Rosenberg on espionage charges. He served more than 18 years in Alcatraz and other federal prisons, traveled to Cuba and Vietnam after his release in 1969 and became an advocate for progressive causes. ... Through it all, he maintained his innocence. But on Thursday, Mr. Sobell, 91, dramatically reversed himself, shedding new light on a case that still fans smoldering political passions. In an interview, he admitted for the first time that he had been a Soviet spy. ....
http://www.nytimes.com/2008/09/12/ny...ll&oref=slogin

While Sobell's interview answers some questions, it leads to other questions; and, in part, the interview is self-serving. A good read, from which you can draw your own conclusions.

The following statement (the "except to corroborate" part) is interesting.

same source
Echoing a consensus among scientists, Mr. Sobell also maintained that the sketches and other atomic bomb details that the government said were passed along to Julius Rosenberg by Ethel’s brother, David Greenglass, were of little value to the Soviets, except to corroborate what they had already gleaned from other moles. ....
We are well aware of other moles (Klaus Fuchs, etc.; and the English Granny mole's activities also have been enhanced by recent "evidence"). However, there was some "evidence" (talk) of a broader US network.

Back in the mid-1950's, I was given a collection of Readers Digests from the 1930's and 1940's. An article ca. 1946-1948 quoted an Air Corps officer, stationed in Alaska, as to boxes of military and technical documents and blueprints which were being shipped to the Russians during the last days of WWII. He looked through the boxes - classified stuff in part; some of which he thought referenced atomic stuff. He wanted to stop the shipment; but since that was the Era of Good Feeling with Uncle Joe, the shipment was shipped.

Now, one cannot necessarily draw conclusions from RD articles. In another of that collection, was an article by some military analysts who concluded that the Japanese were incapable of mounting a major naval attack against our Pacific Fleet, that the Zero and Japanese pilots were equally incompetent; and that the ABCD powers would defeat any Japanese attacks in short order. Its publication date was December 6, 1941 !

Anyway, the Alaskan boxes went into a mental index card, which came out of its rusty file cabinet when I read the "other moles" statement. Back to some harder evidence.

The evidence available in the 1950's has been enhanced by later recovery of data from US and Russian archives - although much data is still probably buried (not necessarily intentionally). Moreover, much of this evidence is difficult. It does not state such things as "Alger Hiss is a GRU agent, who delivered A, B and C today." One has to cross-reference many things to come up with "probable cause" as to the identity of ALES, for example.

That topic (the VENONA intercepts, etc.) is well covered here - a collection of 14 articles; such as:

....
Go to: “Cover Name, Cryptonym, CPUSA Party Name, Pseudonym, and Real Name Index: A Research Historian’s Working Reference”
....
Go to: “Russian Archival Identification of Real Names Behind Cover Names in VENONA,” Center for Cryptologic History Symposium, 27 October 2005
http://www.johnearlhaynes.org/page43.html

The home page has many other links to Cold War "I spy stuff" and CPUSA stuff.

For those who are interesting in "analysing" an actual VENONA intercept (as opposed to what someone says 1822 says), go here:

Center for Cryptologic History Symposium, 27 October 2005
Hiss in VENONA: The Continuing Controversy
John R. Schindler

Of all the historical and political controversies unleashed by the release of the VENONA decrypts nearly a decade ago, none has proved as enduring or vexing as that surrounding Alger Hiss, the U.S. State Department official and reputed Soviet spy. The oft-cited message in question - 1822, Washington to Moscow, dated 30 March 1945 - refers to a well-placed American agent codenamed ALES. .......
http://www.johnearlhaynes.org/page61.html

Some Russian is helpful here (if you feel compelled to verify what Schindler says); but it is not necessary to understand the points he makes. VENONA intercepts I've seen (in the "finished" English - see Schindler for explanation) range from the very clear to the very muddy - many are available on-line with a bit of searching.

Fun stuff - it is interesting how sources and methods take a long time to come out of the woodwork. There is a reason that intelligence services protect (or try to protect) them for a long time.