(Reuters) - The latest sign of trouble for NBC's news division comes from MSNBC, which saw a ratings decline compared to previous weeks as news of the shootings in Colorado was unfolding last Friday.

Not only did MSNBC's ratings significantly trail cable news competitors CNN and Fox News in the prime time period after the shooting, but they were also below the number of viewers the network got on recent Friday nights.

Nielsen ratings data for July 20--the day James Holmes went on a shooting rampage at a Colorado movie theater that killed 12 and injured 58 people--shows that MSNBC's audience during the key prime time hours were below its own average for the previous four Fridays.

MSNBC's total primetime audience--or those who tuned into the network from 8:00 pm to 11:00 pm on July 20--fell 17 percent compared to the previous four Fridays to 575,000 people. During the same timeframe, CNN's audience was up 125 percent at 1.1 million viewers while the No. 1 cable news network Fox News' audience was up 11 percent at 2 million viewers, according to Nielsen research.

Cable news networks generally see a spike in ratings during big breaking news stories such as natural disasters or political upheaval, as viewers seek out the latest developments. For viewers to be turning off MSNBC during a major news event is "disheartening" for NBC, said Horizon media analyst Brad Adgate.

Adgate said the ratings data confirmed the commonly held wisdom among industry observers that "casual news viewers will tune to CNN to find late breaking bulletins and coverage." He noted, for instance, that CNN saw a similar rise with the death of Osama Bin Laden last year.

Following Osama Bin Laden's death in May 2011, CNN drew at least 600,000 more viewers than MSNBC in the evening but still trailed Fox News, according to Nielsen data.

On the day of the shootings in Colorado, MSNBC's ratings decline could also have been the result of viewers tuning into the main NBC broadcast network to watch Ann Curry's return on Friday to report on the shootings, Adgate said.

Curry was unceremoniously ousted from her co-anchor duties on the "Today" show in June shortly after ABC's "Good Morning America" beat it in the morning ratings for the first time in roughly 16 years. Fans of Curry were dismayed by NBC's handling of the situation, and took to social media to voice their support for her.

NBC quickly negotiated a settlement with Curry that paid her a reported $10 million and provided her the opportunity to continue broadcasting on the network.

Another factor that could explain MSNBC's poor showing on July 20 is that the network had replacements in for its top anchors, including Chris Matthews and Rachel Maddow that evening.

Representatives for MSNBC, which is owned by Comcast , and CNN, which is owned by Time Warner Inc, declined to comment.

Founded in 1996, MSNBC blends news and opinion programs that skew liberal and therefore act as a counter to Fox News' conservative leanings. CNN has tried to hold the middle ground in its coverage, a position that some blame for its ratings erosion in recent years. In the second quarter of this year, CNN posted its weakest prime ratings in 21 years while its total viewers also fell 35 percent from a year ago.

Since last September, CNN has trailed MSNBC's average viewership, pulling in an average of 584,000 viewers in primetime compared to MSNBC's 726,000 viewers, according to Nielsen.