By Richard Cowan and Caren Bohan WASHINGTON (Reuters) - Congressional advocates of comprehensive U.S. immigration legislation were diverted into a sometimes testy debate on Monday over whether the measure should be delayed because of questions arising from the Boston Marathon bombing allegedly carried out by two immigrant brothers.

A bipartisan group of US senators finally rolled out an immigration reform bill Thursday, insisting it was the best chance in a generation to fix a broken system, but admitting perils lie ahead. The comprehensive reform effort, filed with the Senate this week, is a huge measure aimed at bringing 11 million undocumented workers out of the shadows and onto a pathway to citizenship, while securing the nation's southern border.

By killing gun reform, the US Senate did more than deal a painfully personal defeat to Barack Obama -- it raised questions about the still ambitious president's entire second term agenda. So far, Obama's thumping November election win has produced little of legislative substance, other than an expiry of tax cuts for the rich. The guns reverse left the president, once the embodiment of change, puzzling over how to get a legacy-boosting program through an obstructive political system already casting an eye to the post-Obama era.

A Texas fertilizer factory exploded in a huge fireball Wednesday, destroying nearby homes and perhaps killing as many as several dozen people, with one official likening the blast to a "nuclear bomb." Smoke and a strong burnt smell lingered in the air hours after the blast in the small town of West, near Waco, and officials expressed fears that toxic fumes could settle over the town. There was also concern that a second fertilizer tank could explode, stoking anxiety in a nation already on edge after the nerve-jangling Boston marathon bombings, which left three dead.