Together more than 30 people were killed in the explosions, putting the city of one million on edge and highlighting the terrorist threat Russia is facing as it prepares to host February's Winter Games in Sochi, President Vladimir Putin's pet project. While terrorists may find it hard to get to the tightly guarded Olympic facilities, the bombings have shown they can hit civilian targets elsewhere in Russia with shocking ease.
Volgograd, located about 650 kilometers (400 miles) northeast of Sochi, serves as a key transport hub for southern Russia, with numerous bus routes linking it to volatile provinces in Russia's North Caucasus, where insurgents have been seeking an Islamic state.
The city, previously called Stalingrad, also serves as an important symbol of Russian pride because of a historic WWII battle in which the Soviets turned the tide against the Nazis.
Vladimir Markin, the spokesman for Russia's main investigative agency said Monday's explosion involved a bomb similar to the one used in Sunday's bombing at the city's main railway station.
"That confirms the investigators' version that the two terror attacks were linked," Markin said in a statement. "They could have been prepared in one place."
Markin said that a suicide attacker was responsible for the bus explosion, reversing an earlier official statement saying that the blast was caused by a bomb that had been left in the vehicle's passenger area.
At least 14 people were killed and nearly 30 were wounded, according to public health officials.
Officials did not name names and no one has claimed responsibility for either bombing, but they came several months after Chechen rebel leader Doku Umarov threatened new attacks against civilian targets in Russia, including Olympics in Sochi in February.
The explosion ripped away much of the bus's exterior and shattered windows in nearby buildings. It virtually paralyzed public transport in the city, forcing many residents to walk long distances to get to work.
Russian authorities have been slow to introduce stringent security checks on bus routes, making them the transport of choice for terrorists in the region. A few months ago authorities introduced a requirement for intercity bus passengers to produce ID when buying tickets, like rail or air passengers, but procedures have remained lax.
Even the tight railway security is sometimes not enough. In Sunday's suicide bombing the attacker detonated in the crowd in front of the station's metal detectors.
Suicide bombings and other terror attacks have rocked Russia for years, but in recent years most of them have been confined to the North Caucasus region. The successive attacks in Volgograd, formerly known as Stalingrad, signaled that militants may be using the transportation hub as a renewed way of showing their reach outside their restive region.
A suicide bus bombing in Volgograd in October killed six people. On Friday, three people were killed when an explosives-rigged car blew up in the city of Pyatigorsk, the center of a federal administrative district created to oversee Kremlin efforts to stabilize the North Caucasus region.
In Sunday's railroad station blast, the bomber detonated explosives just beyond the station's main entrance when a police sergeant became suspicious and rushed forward to check ID, officials said. The officer was killed by the blast, and several other policemen were among some 40 people wounded.
Following Sunday's explosion, the Interior Ministry ordered police to beef up patrols at railway stations and other transport facilities across Russia.
Putin on Monday summoned the chief of the main KGB successor agency and the Interior Minister to discuss the situation, and sent the former to Volgograd to oversee the probe.
Russia in past years has seen a series of terror attacks on buses, trains and airplanes, some carried out by suicide bombers.
Twin bombings on the Moscow subway in March 2010 by female suicide bombers killed 40 people and wounded more than 120. In January 2011, a male suicide bomber struck Moscow's Domodedovo Airport, killing 37 people and injuring more than 180.
Umarov, who had claimed responsibility for the 2010 and 2011 bombings, ordered a halt to attacks on civilian targets during the mass street protests against President Vladimir Putin in the winter of 2011-12. He reversed that order in July, urging his men to "do their utmost to derail" the Sochi Olympics which he described as "satanic dances on the bones of our ancestors."
The International Olympics Committee expressed its condolences over Sunday's bombing in Volgograd, but said it was confident of Russia's ability to protect the Games.
Russian Olympic Committee chief Alexander Zhukov said Monday there was no need to take any extra steps to secure Sochi in the wake of the Volgograd bombings, as "everything necessary already has been done."
Russian authorities have introduced some of the most extensive identity checks and sweeping security measures ever seen at an international sports event.
Anyone wanting to attend the games that open on Feb. 7 will have to buy a ticket online from the organizers and obtain a "spectator pass" for access. Doing so will require providing passport details and contacts that will allow the authorities to screen all visitors and check their identities upon arrival.
The security zone created around Sochi stretches approximately 100 kilometers (60 miles) along the Black Sea coast and up to 40 kilometers (25 miles) inland. Russian forces include special troops to patrol the forested mountains flanking the resort, drones to keep constant watch over Olympic facilities and speed boats to patrol the coast.
The security plan includes a ban on cars from outside the zone from a month before the games begin until a month after they end.
_____ Associated Press writer Jim Heintz contributed to this report.