Manufacturing activity indexes for April released Monday showed a further slowdown across the nation in a confirmation of a statistical trend seen in other data releases last week that points to weaker growth in April.

Perhaps more worryingly, an index of business activity compiled by the Federal Reserve Bank of Dallas turned negative for the first time this year, weighed down by sharply more pessimistic views of growth potential in the future and a highly surprising drop in the number of hours worked by factory workers.

The Dallas Fed's general business activity for the manufacturing sector swung to a negative reading of minus 3.4, the first less-than-zero figure this year, from 10.8 in March. The manufacturing production index was 5.6, less than the 11.1 number noted last month. Those releases come after disappointing business activity data reported by the Chicago PMI Index.  Regional activity indexes compiled by the Philadelphia and New York Federal Reserve branches also underwhelmed analysts.

Referring to the Chicago PMI numbers released earlier in the day, Cooper Howes, an economist at Barclays Capital, wrote in an email to clients that this report mirrored the declines in both the Empire State and Philadelphia Fed earlier this month, and we look for this trend to continue somewhat in the form of a modest decline.

Monday's data releases solidified a risk-off sell-off in U.S. markets, with investors dumping stock in heavy industry and manufacturing, which are seen as most exposed to an economic slowdown at the moment, and parking their cash in safe-haven U.S. Treasuries.

The benchmark S&P 500 Index of U.S. equities slowly but steadily dropped during morning trading in New York, recently quoting at 1,396.62, or 47 basis points off Friday's close. Ten-year government notes rallied and yields, which move opposite price, were as down as 1.915 percent, a reading that approaches mid-February levels for U.S. bonds.