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LM Otero, Pool, Associated Press
An investigator looks over a destroyed fertilizer plant in West, Texas, Thursday, May 2, 2013. Investigators face a slew of challenges in figuring out what caused the explosion at the fertilizer plant that killed 14 people and destroyed part of the small Texas town.

WEST, Texas — Investigators searching for the cause of a massive, deadly fertilizer plant explosion in Texas still don't have a breakthrough.

Two weeks after the April 17 blast that killed at least 14 people at West Fertilizer in the tiny town of West, agents have talked to more than 370 people and received more than 200 tips.

But state Fire Marshal Chris Connealy says Thursday that investigators still haven't found the "critical piece."

Agents are using digital mapping of the plant, rakes, shovels and front-end loaders to sift through dirt and rubble over an approximately 15-acre site. Evidence is being cataloged and tested.

Investigators have not yet searched an approximately 90-foot-wide crater left by the blast. They say the investigation could continue after a previously set May 10 target date.