The Centers for Disease Control and state health officials are tracking down the source of an E. coli outbreak linked to romaine lettuce.

Seventeen cases have been reported in 13 states, including one in Washington.

The illnesses happened November 15 and December 8.

Health officials are also investigating whether the same lettuce caused an E.coli outbreak in several Canadian provinces.

The CDC statement says: "Because we have not identified a source of the infections, CDC is unable to recommend whether U.S. residents should avoid a particular food. This investigation is ongoing, and more information will be released as it becomes available."