New York City ? For the third time in as many years, someone riding a bike and armed with a small explosive has struck in Manhattan, this time in the highest-profile location by far: a landmark military recruitment station in the heart of Times Square.

The bomb produced a sudden flash and billowing cloud of white smoke at about 3:40 a.m. Thursday – a scene captured by numerous security video cameras. There were no injuries, serious damage or clear indication of motive.

But like similar attacks on the British and Mexican consulates, the explosion frayed nerves of New Yorkers and tourists alike. Although authorities have not definitively linked the three attacks, the latest episode heightened speculation that they were the work of a lone bomber.

A law enforcement official said police are investigating letters sent to Capitol Hill offices showing pictures of the recruiting station. According to the official, the letters included words to the effect of, “We did it.”