Flint, Mich., officials are breaking a legal agreement that required them to do an inventory of lead pipes, a group of activists suing the city said.

The plaintiffs in the lawsuit said city officials are using “guesswork” and a “rigged formula” to determine how many homes in the city have lead pipes, according to a June 21 motion.

They are asking a federal judge in the U.S. District Court for the Eastern District of Michigan to, among other things, require the city to hire a statistical expert to change the way Flint evaluates which homes should be prioritized for pipe...