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A trading issue impacted US stock quotes late in the day as Dow flatlined into the close

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The U.S. stock market's consolidated quote system, which feeds real-time data feeds to data providers and is used to calculate index values, experienced a processing issue that resulted in some delayed quotes, according to exchange officials familiar with the matter.

The Nasdaq market flagged an issue affecting the quote system about 24 minutes before the close.

Some traders alerted CNBC to the issue, saying it was affecting the quote for the Dow Jones Industrial Average, which traded in a flat range in about the last 50 minutes of trading. Index values may have been impacted by the processing delay, but all trades were completed appropriately, the sources said.

The New York Stock Exchange said Monday evening that it "continues to experience delays" in the publishing of trades and closing prices to the Consolidated Tape." A number of major exchanges contribute data on their trades to the consolidated quote system, a single feed is run by the Consolidated Tape Association, or CTA, an industry body.

The issue first appeared on the CTA's alerts page around 3:15 p.m. ET Monday. By 3:40 p.m. ET, the problem was reported as "resolved." But just 11 minutes later, the CTA said it was still experiencing a problem.

The CTA also sent an alert to the Nasdaq saying that there was a "critical" issue and that traders were experiencing connectivity and trade input issues into CTS.

After the close the NYSE put out this alert:

"Trading on all NYSE Group exchanges, including closing auctions and real-time submission to DTCC, is functioning normally. Due to the earlier reported CTA issue, there are delays in the publishing of trades and closing prices to the Consolidated Tape for a subset of symbols."

The Chicago Board Options Exchange said Monday evening that "due to today's SIAC CTA issue, official closing prices for certain Cboe listed securities were not properly processed and distributed by SIAC." The Securities Industry Automation Corp. is a subsidiary of the NYSE. 

Nasdaq said all Nasdaq systems are operating normally.

— With reporting by CNBC's Bob Pisani.