CINCINNATI (AP) — First it was the tomatoes. Now it’s the beef. Fourth of July picnic tables are getting a careful look as familiar hamburgers are feared to be among the tainted ingredients in separate food safety scares. On Wednesday, The Kroger Co. expanded its voluntary recall of some ground beef products to its stores in more than 20 states, saying the meat may be contaminated with E. coli. The nation’s biggest traditional grocer also urged customers to check the ground beef in their refrigerators and freezers to determine whether it is covered by the recall. The warning comes as federal investigators try to pinpoint the source of a separate salmonella outbreak linked to tomatoes that has sickened nearly 900 people, raising more questions about the nation’s food safety system. While insisting that tomatoes remain the leading suspect, investigators are looking at other produce but remain mum on exactly what vegetables are getting tracked.