September was the most violent month in the last year and a half in terms of rock throwing in the West Bank, Israel Defense Forces figures reveal.

There were 498 incidents of rocks being thrown last month - 33 percent more than the monthly average over the past year. This was the highest monthly total since Operation Cast Lead in Gaza in early 2009.

Rock throwing also took the lives of two Israelis last month: Asher Palmer and his infant son Yonatan, who were killed in an incident in which rocks are believed to have been thrown from a passing car.

There were 3,484 incidents of rock throwing against IDF forces and against passing cars in the West Bank in the first nine months of 2011 - an average of 387 per month. This is way up from 303 per month in 2010.

But the differences between months are great: For example, October 2010 was the quietest month with 133 incidents, while November and December last year had 290 incidents each. January saw a jump to 474 such incidents.

A senior officer in the IDF's Central Command said that after Palestinian President Mahmoud Abbas' speech at the United Nations, "There was a large increase in stone throwing, it seems because of his mention of the settlers."

Looking at recent years, there has been an average of 4,066 such incidents a year since 2004, with 2007 being the quietest, with 3,501 incidents. The most violent year in terms of rock throwing was 2005, with 4,371 incidents. But if the present pace continues in 2011, this year will have seen the most incidents.

Despite the detailed numbers provided by the IDF on stone-throwing incidents, police have no data on the number of injuries caused by it. Recently, the B'Tselem human rights group asked the police for information on such injuries, applying under the freedom of information law. In response, the police wrote: "There is no computerized system with the data. The analysis requires an examination of hundreds of documents in every file."

The Shin Bet security service has taken over the investigation into the deaths of Asher and Yonatan Palmer, who were killed two weeks ago after their car rolled over after being attacked with rocks near Halhul. The Shin Bet does not usually deal with rock-throwing cases, but decided to in this instance due to the fatal results and the assumption that the rocks were thrown from a passing car.

The police has evidence of 18 similar cases of rocks being thrown from vehicles over the past few months, but the IDF said it knew of only three such incidents.

Rabbi Yaron Durani of Nokdim was attacked on his way from Gush Etzion, south of Jerusalem, to Nokdim when rocks were thrown from a passing vehicle just before Rosh Hashanah.