Prime Minister Benjamin Netanyahu said Israel would find those who committed the bus bombing in Jerusalem. The police were still investigating who was behind it.  JERUSALEM — A bomb exploded on a bus in Jerusalem on Monday, wounding about 21 people and feeding a sense of vulnerability among many Israelis after months of simmering violence.  The sights and sounds were familiar: Television images of a burned-out hulk of a bus and wailing sirens, immediately reminding many Israelis of the second Palestinian uprising, which erupted in 2000, when suicide bombers blew up buses in Jerusalem and other Israeli cities, killing scores. Attacks on buses have been rare in recent years.  Describing the blast as a terrorist attack, Prime Minister Benjamin Netanyahu, whose image as Israel’s security czar has been dented in recent months, vowed to track down those responsible.  “We will find whoever prepared that explosive device, we will get to those who sent it and we will also get to those who stand behind them,” he said, adding, “We will settle accounts with those terrorists.”  The bombing came as many Israelis were already on edge after a six-month wave of stabbings, shootings and vehicle attacks by Palestinians that have killed about 30 people. More than 200 Palestinians have been killed by Israeli forces during that period, according to the Palestinian Health Ministry. Israeli officials say that most of the Palestinians were killed while carrying out, or attempting, attacks, and that others were killed in clashes with Israeli security forces.  Unlike the major assaults of the second uprising, which were engineered by the main Palestinian groups, most of the recent attacks appear to be the almost spontaneous work of by individuals.  “We are in an ongoing battle against terrorism,” Mr. Netanyahu said, “terrorism of knives, terrorism of shootings, of bombs, of rockets and even of tunnels.”  He was referring to a tunnel that the Israeli military said on Monday it had discovered and “neutralized.” Running from the Gaza Strip into Israeli territory, it was the first such tunnel found since a cease-fire ended 50 days of fighting in the Palestinian coastal territory in summer 2014.  Mr. Netanyahu hailed the discovery, saying, “the State of Israel has achieved a global breakthrough in the ability to locate tunnels.” He described the breakthrough as “unique,” but did not provide any details about the technology involved. The military said the tunnel had been built 100 to 130 feet underground by Hamas, the Islamic militant group that controls Gaza; that it could have been used for terrorist attacks against Israeli border communities; and that it was equipped with electricity and communications lines.  Israeli officials have said in recent months that Hamas had most likely rebuilt much of the tunnel network that was damaged by Israel during the 50-day war. The officials also said Hamas was trying to restock its rocket supplies.  The threat of the tunnels continues to sow fear in Israeli communities along the border. Some residents have reported hearing digging underground.  Feeding those anxieties, Hamas’s military wing said in a statement that the recently discovered tunnel “is only a drop in the ocean of resistance preparations” meant to defend the Palestinians.  Hamas also praised the Jerusalem bus bombing on Monday, without claiming responsibility for it. “We bless the Al-Quds operation,” the organization wrote on its Twitter account, using the Arabic name for Jerusalem.  The details of the bus explosion on Monday emerged slowly. The blast came shortly before 6 p.m. in Moshe Baram Street, by the Talpiot industrial zone in southeast Jerusalem. A fire caused by the explosion spread quickly to a second bus, but the police said it was not immediately clear whether the blaze was the result of a bomb or caused by a mechanical flaw.  More than two hours later, Yoram Halevy, the Jerusalem district police chief, said in a televised statement that a bomb had caused the blast, but the police were cautious about determining if it involved terrorism, or crime.  Mr. Halevy said the police were still investigating the circumstances surrounding the blast, including whether one of the people who was wounded had brought the bomb onto the bus. Two victims were in serious condition. The identity of at least one of the wounded was still unknown hours after the blast, Micky Rosenfeld, a police spokesman, said.  Mr. Rosenfeld said the police were investigating how the bomb got onto the bus, and if it was meant to go off there or was intended for another location.  Racheli Dadon, a Jerusalem resident, described from her hospital bed how she had boarded the No. 12 bus with her daughter, Eden, 15. “I had not even sat down when I heard a huge blast and all the glass fell on us,” she told the Israelie news site Ynet. She said she searched for her daughter amid smoke and darkness, finding her with burns all over her body.  The driver of the No. 12 bus, Moshe Levy, said he had inspected the vehicle twice before setting out on his route. “The whole way was fine,” he said. “I got to Baram Street in Talpiot and stopped in a traffic jam. I heard a blast in the back portion of the bus and I opened the doors and told people to exit the bus.”  Prof. Avraham Rivkind, head of the trauma unit at the Hadassah Medical Center in Ein Kerem, on the southwestern edge of Jerusalem, said the hospital received several of the wounded who had suffered burns as well as injuries from nails and bolts — “wounds,” he said, “that are familiar to us from past events in our city.”  Also on Monday, Israeli military prosecutors filed a manslaughter charge against a soldier who fatally shot a Palestinian assailant in the West Bank city of Hebron last month as he lay on the ground, wounded and subdued. The military court lifted an order banning the publication of the name of the soldier, Sgt. Elor Azaria, from Ramle, in central Israel, and said his trial, in a military court, would begin on May 9.  Follow Isabel Kershner on Twitter @IKershner  Majd Al Waheidi contributed reporting from Gaza and Diaa Hadid from Jaffa, Israel  A version of this article appears in print on April 19, 2016, on page A8 of the National edition with the headline: Bus Bombing in Jerusalem Reawakens Familiar Anxieties After Months of Violence. Order Reprints| Today's Paper|Subscribe
The bombing on Monday afternoon started a fire that spread quickly from one bus to another. Two of the wounded were listed in serious condition, the authorities said.