You seem to be implying that Newsweek is acting irresponsibly. On the contrary, they appear to be reporting the most current information available at the time of the publication.
The information put out by the British government has been highly contradictory from day to day. Initially they suggested that the bombs were crude devices, then that they were of military quality, then that they used homemade explosives. I read a report similar to Newsweek's in the Times of London published a day before the Newsweek article.
Terrorist gang 'used military explosives'