(CNN) Belgian authorities are searching for two suspects who escaped when police raided a Brussels apartment linked to last year's terror attacks in Paris.

The apartment raid turned up an ISIS flag, a Kalashnikov rifle and ammunition.

But two people escaped after an intense, prolonged firefight, according to a spokesman for the Belgian federal prosecutor's office.

During the raid Tuesday, authorities killed suspect Belkhaid Mohammed, 35. The Algerian had not been on authorities' radar before, spokesman Eric Van der Sypt said in a statement.

Photos: World reacts to Paris attacks Photos: World reacts to Paris attacks President Barack Obama, French President Francois Hollande, second from right, and Paris Mayor Anne Hidalgo arrive at the Bataclan, site of one of the Paris terrorists attacks, to pay their respects to the victims after Obama arrived in town for the COP21 climate change conference early on Monday, November 30, in Paris. Hide Caption 1 of 40 Photos: World reacts to Paris attacks The Eiffel Tower in Paris is illuminated in the French national colors on Monday, November 16. Displays of support for the French people were evident at landmarks around the globe after the deadly terrorist attacks in Paris on Friday, November 13. Hide Caption 2 of 40 Photos: World reacts to Paris attacks People hold hands as they observe a minute of silence in Lyon, France, on November 16, three days after the Paris attacks. A minute of silence was observed throughout the country in memory of the victims of the country's deadliest violence since World War II. Hide Caption 3 of 40 Photos: World reacts to Paris attacks French President Francois Hollande, center, flanked by French Prime Minister Manuel Valls, right, and French Education Minister Najat Vallaud-Belkacem, center left, stands among students during a minute of silence in the courtyard of the Sorbonne University in Paris on November 16. Hide Caption 4 of 40 Photos: World reacts to Paris attacks A large crowd gathers to lay flowers and candles in front of the Carillon restaurant in Paris on Sunday, November 15. Hide Caption 5 of 40 Photos: World reacts to Paris attacks A man sits next to candles lit as homage to the victims of the deadly attacks in Paris at a square in Rio de Janeiro on November 15. Hide Caption 6 of 40 Photos: World reacts to Paris attacks People light candles in tribute to the Paris victims on November 15 in Budapest, Hungary. Hide Caption 7 of 40 Photos: World reacts to Paris attacks People gather outside Notre Dame Cathedral in Paris on November 15 for a national service for the victims of the city's terror attacks. Hide Caption 8 of 40 Photos: World reacts to Paris attacks People write messages on the ground at Place de la Republique in Paris on November 15. Hide Caption 9 of 40 Photos: World reacts to Paris attacks People pray during a candlelight vigil for victims of the Paris attacks at a church in Islamabad, Pakistan, on November 15. Hide Caption 10 of 40 Photos: World reacts to Paris attacks French golfer Gregory Bourdy passes a peace symbol for the Paris victims during the BMW Shanghai Masters tournament November 15 in Shanghai, China. Hide Caption 11 of 40 Photos: World reacts to Paris attacks A man offers a prayer in memory of victims of the Paris attacks at the French Embassy in Tokyo on November 15. Hide Caption 12 of 40 Photos: World reacts to Paris attacks A woman holds a candle atop a miniature replica of the Eiffel Tower during a candlelight vigil Saturday, November 14, in Vancouver, British Columbia. Hide Caption 13 of 40 Photos: World reacts to Paris attacks Front pages of Japanese newspapers in Tokyo show coverage and photos of the Paris attacks on November 14. Hide Caption 14 of 40 Photos: World reacts to Paris attacks An electronic billboard on a canal in Milan, Italy reads, in French, "I'm Paris," on November 14. Hide Caption 15 of 40 Photos: World reacts to Paris attacks The Eiffel Tower stands dark as a mourning gesture on November 14, in Paris. More than 125 people were killed in a series of coordinated attacks in Paris on Friday. People around the world reacted in horror to the deadly terrorist assaults. Hide Caption 16 of 40 Photos: World reacts to Paris attacks Lithuanians hold a candlelight vigil in front of the French Embassy in Vilnius, Lithuania, on November 14. Hide Caption 17 of 40 Photos: World reacts to Paris attacks Thousands gather in London's Trafalgar Square for a candlelit vigil on November 14 to honor the victims of the Paris attacks. Hide Caption 18 of 40 Photos: World reacts to Paris attacks A woman lights candles at a memorial near the Bataclan theater in Paris on November 14. Hide Caption 19 of 40 Photos: World reacts to Paris attacks A man places a candle in front of Le Carillon cafe in Paris on November 14. Hide Caption 20 of 40 Photos: World reacts to Paris attacks A woman holds a French flag during a gathering in Stockholm, Sweden, on November 14. Hide Caption 21 of 40 Photos: World reacts to Paris attacks Nancy Acevedo prays for France during the opening prayer for the Sunshine Summit being held at Rosen Shingle Creek in Orlando, Florida on November 14. Hide Caption 22 of 40 Photos: World reacts to Paris attacks French soldiers of the United Nations' interim forces in Lebanon observe the national flag at half-staff at the contingent headquarters in the village of Deir Kifa on November 14. Hide Caption 23 of 40 Photos: World reacts to Paris attacks A couple surveys the signature sails of the Sydney Opera House lit in the colors of the French flag in Sydney on November 14. Hide Caption 24 of 40 Photos: World reacts to Paris attacks A woman places flowers in front of the French Consulate in St. Petersburg, Russia, on November 14. Hide Caption 25 of 40 Photos: World reacts to Paris attacks Candles are lit in Hong Kong on November 14 to remember the scores who died in France. Hide Caption 26 of 40 Photos: World reacts to Paris attacks A woman lights a candle outside the French Consulate in Barcelona, Spain, on November 14. Hide Caption 27 of 40 Photos: World reacts to Paris attacks Britain's Prince Charles expresses solidarity with France at a birthday barbecue in his honor near Perth, Australia, on November 14. Hide Caption 28 of 40 Photos: World reacts to Paris attacks The French national flag flutters at half-staff on November 14 at its embassy in Beijing. Hide Caption 29 of 40 Photos: World reacts to Paris attacks Dutch Prime Minister Mark Rutte after a speech on November 14 in The Hague following the attacks. Hide Caption 30 of 40 Photos: World reacts to Paris attacks Japanese Prime Minister Shinzo Abe becomes emotional after his speech on the French attacks during the opening ceremony of a Japanese garden in Istanbul, Turkey, on November 14. Hide Caption 31 of 40 Photos: World reacts to Paris attacks A woman mourns outside Le Carillon bar in the 10th district of Paris on November 14. The attackers ruthlessly sought out soft targets where people were getting their weekends underway. Hide Caption 32 of 40 Photos: World reacts to Paris attacks People lay flowers outside the French Embassy in Moscow on November 14. Hide Caption 33 of 40 Photos: World reacts to Paris attacks Mourners gather outside Le Carillon bar in the 10th district of Paris on November 14. "We were listening to music when we heard what we thought were the sounds of firecrackers," a doctor from a nearby hospital who was drinking in the bar with colleagues told Le Monde. "A few moments later, it was a scene straight out of a war. Blood everywhere." Hide Caption 34 of 40 Photos: World reacts to Paris attacks People attend a vigil outside the French Consulate in Montreal. Prime Minister Justin Trudeau offered "all of Canada's support" to France on Friday, November 13, in the wake of the attacks. Hide Caption 35 of 40 Photos: World reacts to Paris attacks Police show a heightened presence in Times Square in New York on November 13, following the terrorist attacks in Paris. Hide Caption 36 of 40 Photos: World reacts to Paris attacks People light candles at a vigil outside the French Consulate in Montreal on November 13. Hide Caption 37 of 40 Photos: World reacts to Paris attacks University of Nevada, Las Vegas, fans observe a moment of silence for the victims of the terrorist attacks in Paris before a basketball game November 13. Hide Caption 38 of 40 Photos: World reacts to Paris attacks The house lights are shut off and scoreboard dark as Boston Celtics players pause for a moment of silence for the Paris victims before an NBA basketball game against the Atlanta Hawks in Boston on November 13. Hide Caption 39 of 40 Photos: World reacts to Paris attacks People light candles at a vigil outside the French Consulate in Montreal on November 13. Hide Caption 40 of 40

A sniper for the Belgian special forces killed Mohammed when he tried to open fire on police from a window.

His body was found next to a rifle and a book on Salafism, a puritanical branch of Islam that dictates only the followers of the Prophet Mohammed practice the correct Islam.

Authorities also discovered 11 loaded Kalashnikov magazines "and innumerable shell casings," but no explosives, according to Van der Sypt.

An intense manhunt followed Tuesday's operation in the southern Brussels neighborhood of Forest. Authorities connected it to the investigation of the Paris terror attacks that killed 130 people in November.

What happened

The raid Tuesday started when four Belgian and two French police officers arrived at what they thought was an empty residence in Forest, according to a senior Belgian counterterrorism official.

"From the moment that the door of the flat was opened, at least two persons ... opened fire toward them," Van der Sypt said.

Photos: Political violence is driving terrorism Photos: Political violence is driving terrorism A Nigerian policeman inspects the site of a suicide attack by Boko Haram at a busy cattle market in the northeastern Nigerian city of Maiduguri in June, 2015. Boko Haram overtook ISIS as the world's deadliest terror group last year, according to the Global Terrorism Index, while Nigeria had the biggest year-on-year increase in terrorism, with deaths up more than 300%. Hide Caption 1 of 6 Photos: Political violence is driving terrorism In this image taken from social media, an ISIS fighter holds the group's flag as he stands on a tank, purportedly captured when they took over the town of Qaryatain, Syria. ISIS, along with Boko Haram, is one of two groups accounting for 51% of claimed terrorism deaths in 2014. Hide Caption 2 of 6 Photos: Political violence is driving terrorism A member of the Iraqi security forces fires ammunition in Baghdad in 2014 during a funeral procession of an Iraqi politician. The deadliest city in the world for terrorism is Baghdad. There were 2,454 deaths in Baghdad in 2014, with a death rate from terrorism of 43 per 100,000 people. Hide Caption 3 of 6 Photos: Political violence is driving terrorism Iraqi counter-terrorism forces patrol a street in Tikrit in April 2015, a day after the country's prime minister declared victory in the battle to retake the city from ISIS. Hide Caption 4 of 6 Photos: Political violence is driving terrorism An Iraqi Kurdish Peshmerga fighter prepares an ammunition belt as he guards a position at the frontline of fighting against ISIS militants near the northern Iraqi town of Sinjar. Hide Caption 5 of 6 Photos: Political violence is driving terrorism A Syrian family waits after arriving on the Greek island of Lesbos along with other migrants and refugees, on November 17, 2015. Ten of the 11 countries most affected by terrorism also have the highest rates of refugees. Hide Caption 6 of 6

Three officers -- including a Frenchwoman -- were hurt in the initial exchange of gunfire before managing "to retreat safely," he said. A fourth officer from Belgium suffered a slight injury to the head in a later exchange.

Reinforcements arrived, with bullets flying back and forth for several hours.

There's no change in Belgium's threat level, now at its second highest. But Prime Minister Charles Michel acknowledged the raid underscored the need to be on guard.

"The terrorist threat level is maintained at level three as yesterday's events confirm the threat is still real," he said.

Belgium a focus

It's unclear what connection Tuesday's raid has to the November 13 carnage in Paris. Belgium has long been a focal point for investigators.

Photos: Night of terror: Paris attacks Photos: Night of terror: Paris attacks A forensic scientist works near a Paris cafe on Saturday, November 14, following a series of coordinated attacks in Paris the night before that killed scores of people. ISIS has claimed responsibility. Hide Caption 1 of 32 Photos: Night of terror: Paris attacks Police are out in force November 14 near La Belle Equipe, one of the sites of the terror attacks. Hide Caption 2 of 32 Photos: Night of terror: Paris attacks Forensic police search for evidence inside the Comptoir Voltaire cafe after the attacks. Hide Caption 3 of 32 Photos: Night of terror: Paris attacks Shoes and a bloody shirt lie outside the Bataclan concert hall on November 14. Most of the fatalities occurred at the Bataclan in central Paris. Hide Caption 4 of 32 Photos: Night of terror: Paris attacks Security forces evacuate people on Rue Oberkampf near the Bataclan concert hall early on November 14. Hide Caption 5 of 32 Photos: Night of terror: Paris attacks Medics evacuate an injured woman on Boulevard des Filles du Calvaire near the Bataclan early on November 14. Hide Caption 6 of 32 Photos: Night of terror: Paris attacks Police, firefighters and rescue workers secure the area near the Bataclan concert hall on November 14. Hide Caption 7 of 32 Photos: Night of terror: Paris attacks A man with blood on his shirt talks on the phone on November 14. He is next to the Bataclan theater, where gunmen shot concertgoers and held hostages until police raided the building. Hide Caption 8 of 32 Photos: Night of terror: Paris attacks Police officers patrol the area around Notre Dame cathedral in Paris on November 14. Hide Caption 9 of 32 Photos: Night of terror: Paris attacks Police officers patrol Paris' Saint-German neighborhood on November 14. Hide Caption 10 of 32 Photos: Night of terror: Paris attacks Victims of the shooting at the Bataclan concert venue in central Paris are evacuated to receive medical treatment on November 14. Hide Caption 11 of 32 Photos: Night of terror: Paris attacks A woman is evacuated from the Bataclan theater early on November 14. Hide Caption 12 of 32 Photos: Night of terror: Paris attacks Forensics are working in the street of Paris after the terrorist attack on Friday, November 13. The words "horror," "massacre" and "war" peppered the front pages of the country's newspapers, conveying the shell-shocked mood. Hide Caption 13 of 32 Photos: Night of terror: Paris attacks Rescuers evacuate an injured person near the Stade de France, one of several sites of attacks November 13 in Paris. Thousands of fans were watching a soccer match between France and Germany when the attacks occurred. Hide Caption 14 of 32 Photos: Night of terror: Paris attacks A survivor of the terrorist attack in the Bataclan is assisted following terror attacks, November 13. The violence at the Bataclan, which involved a hostage-taking, resulted in the highest number of casualties of all the attacks. Hide Caption 15 of 32 Photos: Night of terror: Paris attacks Spectators invade the pitch of the Stade de France stadium after the international friendly soccer match between France and Germany in Saint-Denis. Hide Caption 16 of 32 Photos: Night of terror: Paris attacks Spectators embrace each other as they stand on the playing field of the Stade de France stadium at the end of a soccer match between France and Germany in Saint-Denis, outside Paris, on November 13. Hide Caption 17 of 32 Photos: Night of terror: Paris attacks A body, covered by a sheet, is seen on the sidewalk outside the Bataclan theater. Hide Caption 18 of 32 Photos: Night of terror: Paris attacks Rescuers evacuate an injured person on Boulevard des Filles du Calvaire, close to the Bataclan concert hall in central Paris. Hide Caption 19 of 32 Photos: Night of terror: Paris attacks Wounded people are evacuated outside the scene of a hostage situation at the Bataclan theater in Paris on November 13. Hide Caption 20 of 32 Photos: Night of terror: Paris attacks A medic tends to a wounded man following the attacks near the Boulevard des Filles du Calvaire. Hide Caption 21 of 32 Photos: Night of terror: Paris attacks A woman walks past police and firefighters in the Oberkampf area of Paris. Hide Caption 22 of 32 Photos: Night of terror: Paris attacks A riot police officer stands by an ambulance near the Bataclan concert hall in central Paris. Hide Caption 23 of 32 Photos: Night of terror: Paris attacks Wounded people are evacuated outside the Bataclan concert hall. Hide Caption 24 of 32 Photos: Night of terror: Paris attacks Police secure the Stade de France in Saint-Denis, north of Paris, following explosions during the soccer match between France and Germany. Hide Caption 25 of 32 Photos: Night of terror: Paris attacks A wounded man is evacuated from the Stade de France in Saint-Denis, outside Paris. Hide Caption 26 of 32 Photos: Night of terror: Paris attacks Spectators gather on the field of the Stade de France after the attacks. Explosions were heard during the soccer match between France and Germany. Hide Caption 27 of 32 Photos: Night of terror: Paris attacks French security forces rush in as people are evacuated in the area of Rue Bichat in the 10th District of Paris. Hide Caption 28 of 32 Photos: Night of terror: Paris attacks People leave the Stade de France after explosions were heard near the stadium during a soccer match between France and Germany on Friday. Paris Deputy Mayor Patrick Klugman told CNN President Francois Hollande was at the match and was evacuated at halftime. Hide Caption 29 of 32 Photos: Night of terror: Paris attacks Victims lay on the pavement outside a Paris restaurant. Hide Caption 30 of 32 Photos: Night of terror: Paris attacks Rescue workers and medics tend to victims at the scene of one of the shootings, a restaurant in the 10th District. Attackers reportedly used AK-47 automatic weapons in separate attacks across Paris, and there were explosions at the Stade de France. Hide Caption 31 of 32 Photos: Night of terror: Paris attacks French security forces move people in the area of Rue Bichat in the 10th District. A witness told BFMTV that firefighters were on the scene to treat the injured. Hide Caption 32 of 32

Two terrorist operatives phoned in orders from Brussels to those directly involved in the Paris attacks, according to a senior Belgian counterterrorism official.

These two had an even more integral role than Abdelhamid Abaaoud, the man long identified as the ringleader of the attacks, according to the official.

Yet others with Belgian connections and ties to the November attacks remain at large.

They include Salah Abdeslam, a Belgian-born French national who lived and spent time with Abaaoud in a Belgian prison. The trail for Abdeslam, one of the few alleged Paris attackers to escape alive, went cold in December, according to a senior European counterterrorism official.

Photos: Paris attack suspects: What we know Photos: Paris attack suspects: What we know Hide Caption 1 of 10 Photos: Paris attack suspects: What we know Hide Caption 2 of 10 Photos: Paris attack suspects: What we know Hide Caption 3 of 10 Photos: Paris attack suspects: What we know Hide Caption 4 of 10 Photos: Paris attack suspects: What we know Hide Caption 5 of 10 Photos: Paris attack suspects: What we know Hide Caption 6 of 10 Photos: Paris attack suspects: What we know Hide Caption 7 of 10 Photos: Paris attack suspects: What we know Hide Caption 8 of 10 Photos: Paris attack suspects: What we know Hide Caption 9 of 10 Photos: Paris attack suspects: What we know Hide Caption 10 of 10

French sources close to the investigation said Abdeslam was not the target of Tuesday's raid.

Many of those tied to the Paris massacre lived in Belgium, and they're believed to have met there before planning attacks.