Security forces have received a number of warnings of possible terror attacks in the West Bank and within the Green Line by Hamas activists recently released from Israeli jails, a senior Israel Defense Forces officer said over the weekend.

The IDF and the Shin Bet security service have seen efforts by Hamas over the past few months to rehabilitate its military infrastructure in the West Bank. The organization was hit hard by a wave of arrests and targeted killings undertaken by Israel between 2001 and 2005.

Previously, in the intifada's early years, Hamas had been dominant in terms of the scope of terror attacks it initiated. The balance of power shifted due to the damage Hamas suffered, and since 2004, Islamic Jihad, particularly its network in Jenin and Tul Karm, has become the most dangerous group in the territories.

Now the picture has changed again, thanks to two developments: Islamic Jihad itself has been hit hard in recent years, and a large number of lower-level Hamas activists have recently been released from Israeli jails after serving sentences of about five years for relatively minor intifada-related crimes.

An unknown but apparently large number of these ex-convicts return to terrorism, employing new techniques picked up on the inside from veteran prisoners. The army and the Shin Bet say that most of these activists are quick to begin setting up new networks to carry out major attacks.

On the recommendation of the army and the Shin Bet, the Prison Service is now making efforts to keep prisoners from the West Bank separate from those from Gaza, and also to keep Hamas inmates apart from members of other groups, since Hamas tends to exert influence on prisoners from other organizations, including Fatah.

Security officials say Hamas networks are springing up mainly in the area of Hebron and Nablus. A return to terror does not contradict Hamas policy, since the cease-fire agreement between Hamas and Israel, which was instituted on June 19, applies to the Gaza Strip only, and not the West Bank.

Hamas seems to have an interest in continuing attacks in the West Bank to prove to the Palestinian public that it has not completely abandoned the armed struggle against Israel. Security forces are also trying to find out whether Hamas is linked in any way to the three attacks in recent months in Jerusalem (the shooting at Mercaz Harav Yeshiva and the two bulldozer attacks in the capital's center). So far no real connection has been found between the three perpetrators of those attacks.

The understanding that released prisoners are returning to terror also has significance for the talks over the release of kidnapped soldier Gilad Shalit. One of the main arguments the Shin Bet has raised against the release of murderers is the concern that they will continue where they left off when let out of jail. Although there are no official statistics on the rate of terrorist recidivism, it is clearly high.

The most recent terror alerts, like the re-arrest of released prisoners, shows that the question is not only about what crimes convicts committed in the past, but also the extent of danger posed they are believed to pose now. Experts, some within the security services, claim that the greatest risk is sometimes posed by those who committed relatively minor offenses, but then obtained know-how and motivation from other prisoners during their incarceration.