A total 150 people were killed in Mexico's Ciudad Juarez in May, the lowest monthly number since March 2009, when a massive army deployment caused the murder rate to temporarily plummet.

Before the recent improvement, the border town, across the Rio Grande from El Paso, Texas, had enjoyed the dubious designation of the world’s most dangerous city for some years. At its peak of violence, 350 people were killed in October 2010, in the city of 1.3 million or so residents. Since then, the violence has steadily declined, slipping to 183 murders in March and 168 in April.

Despite the dip in killings, Juarez remains an exceedingly violent city; the May total may have been the lowest since 2009, but it was enough to put the annualized murder rate well above 100 per 100,000 residents, which would make it the bloodiest of any major Mexican metropolis.

President Felipe Calderon previously hailed the work of the Federal Police in bringing about the decline in violence, though he offered no explanation as to how this had taken place.

One likely factor is attrition among the hundreds of gangs operating in Juarez. In what might be a sign of the shifting dynamics in the regional underworld, two larger groups that operate in Chihuahua -- the Zetas and La Linea -- recently announced an alliance in the city of Hidalgo del Parral, a few hours south of Juarez.