MEXICO CITY - The U.S. has cleared tomatoes in 28 of 31 Mexican states from suspicion in a recent salmonella outbreak, a move that drew praise from Mexican officials Saturday.

The U.S. Food and Drug Administration now appears to be narrowing its investigation into the outbreak, which has sickened more than 550 people, to some counties in Florida and three Mexican states: Jalisco, Coahuila and Sinaloa.

The decision represents "important progress" and was the result of "a day of intense negotiations between (Mexican) federal authorities and the FDA," Mexico's Agriculture Department said in a press statement.

Mexico's tomato exports, a big source of income for farmers, were all but halted by the salmonella outbreak, which began in April.

Some cases are still being reported.

U.S. warnings against consuming Mexican tomatoes caused resentment here.

Mexican officials said they'll continue to work to prove that the salmonella detected in the United States did not originate in their country.

The FDA inspection in Mexico is now looking at some distributors who were handling tomatoes in the western state of Jalisco and the northern state of Coahuila when the outbreak occurred.

"However, there is no evidence at this time that they were related" to the outbreak, according to the press statement.

While Sinaloa is Mexico's most important tomato-growing state and remains on the warning list, it has ended its tomato-growing season for the year.

FDA investigators have been tracking where those stricken with salmonella said they bought or ate tomatoes, and where the retailers or restaurants bought them.

They have zeroed in on a list of farms in Florida and Mexico that seem to have contributed at least partly to the supply, plus records showing the packing houses and other distribution stops between the farms and markets.

FDA inspectors will be involved in reviews of the suspected sites.