WASHINGTON — More than seven weeks after the start of a massive E. coli illness outbreak from romaine lettuce that sickened 172 people and caused romaine sales to plummet 45 percent, the Food and Drug Administration says it has no idea who or what caused the contamination.

Agency investigators have not managed to trace the affected lettuce back to one farm, processor, or distributor, FDA Commissioner Scott Gottlieb said in an update Thursday. And with the affected lettuce now off shelves and the growing season over, there’s a chance the FDA may never crack the case.

The mystery has frustrated consumer advocates, who have called on the FDA to issue rules that would speed up future foodborne illness investigations. The outbreak, which began April 10, is the largest flare-up of E. coli in a decade.

On Thursday, the FDA indicated it will consider a range of reforms to improve its ability to trace the source of future outbreaks.

‘‘We want American consumers to be confident in the quality and safety of the lettuce they consume,’’ Gottlieb said in a statement.

WASHINGTON POST