An index tracking U.S. economic activity fell in October to its lowest level since the last recession, the Federal Reserve Bank of Chicago said on Wednesday.

The three-month moving average of the Chicago Fed's National Activity Index plunged to -1.56 in October from -1.06 in September, even as the monthly reading inched up to -1.70 last month from September's -1.78. 

The latest reading on the three-month average, considered the more reliable gauge since it smooths out monthly fluctuations, was its lowest since March 1991, when the U.S. economy began its climb out of its last recession. In each of the five recessions since 1967, the year the index measures back to, the three-month average fell below -1.50. 

"The negative October reading is continuing evidence that national economic growth was substantially below trend and the U.S. economy is in recession," the regional Fed bank said. 

It was the 16th straight month that the three-month average was below zero, which the Chicago Fed said suggested economic activity has been growing below trend for more than a year. 

Last week, the National Bureau of Economic Research's business cycle dating committee declared that a U.S. recession began in March. The six-member committee is considered the final arbiter of U.S. recessions. 

The Chicago Fed index is based on 85 economic indicators that cover areas including production, income, employment, consumption, housing and manufacturing. Fifty-seven of the 85 indicators showed below-average growth in October, the Chicago Fed said. 

In October 2000, the monthly index was -0.64 while the three-month moving average was -0.41.