JERUSALEM (AP) - Israel has instituted procedures for preventing suicide bombings on buses after 20 such attacks during four years of Palestinian-Israeli violence.

Security officers are stationed in Jerusalem, Tel Aviv and other cities to search bus stops and buses for suspicious people and objects. Sometimes they use dogs to detect explosives.

The security officers have been trained to spot suicide attackers. The officers frequently address passengers with a simple "hello" in Hebrew, looking them directly in the eye. Those who do not answer are considered suspect and can be questioned further.

Israeli bus drivers have been briefed on how to identify suicide bombers. A few buses have been fitted with a special system to detect explosives among passengers waiting to get on the bus. A turnstile at the bus door can be locked to keep a potential attacker off.

Central bus stations in main cities, often combined with shopping malls, are carefully guarded. People entering the stations pass through metal detectors, and their bags are examined in airport-style X-ray devices.

At bus entrances to the main terminals, security officers examine each vehicle, sometimes boarding a bus to look at each passenger and check for suspicious packages.

Of the 119 Palestinian suicide bombings since the current round of violence began in 2000, 20 were on Israeli buses, killing 208 passengers. In the most recent attack, two bomber blew themselves up simultaneously on buses in the southern Israeli city of Beersheba on Aug. 31 last year, killing 16.