Police have raised to six the number of people they believe were killed in an apparently random shooting spree around the city of Washington on Thursday.

Bullet fragments found at the scene of the sixth killing matched those from at least three of the five slayings initially blamed on the random shooter, authorities said.

Police are hunting for a sniper and a driver accomplice.

The sixth victim - 72-year-old Pascal Charlot - was killed by a single shot while walking on a street corner in the US capital, close to its border with Maryland.

He was shot at 2115 on Thursday, nearly 12 hours after the 15-hour period when the other five shootings occurred, all with a five-mile (eight-kilometre) radius in Maryland's Montgomery County.

Three men and two women were shot and killed with high-velocity bullets in the separate incidents - two at petrol stations and one outside a post office in an affluent suburb of Washington DC.

You've got a driver, you've got a shooter

Charles Moose Local police chief

Police said the victims were hit "from a distance," probably with .223-calibre bullets from an assault or hunting rifle.

Nearly 150 state troopers and federal agents - including the FBI and Secret Service - are investigating the case and the local authorities have offered a reward of $50,000 for information leading to an arrest and conviction.

No obvious pattern

The police believe the killings are linked, they are struggling to find any connection between them.

"There is not a lot of indication at this time that any of these individuals know each other," said Montgomery Country police chief Charles Moose.

One victim was shot dead while sitting on a bench

One victim was shot dead while riding a lawnmower, another while cleaning her car at a petrol station.

None of the six appeared to have been robbed.

Police also said race did not seem to be a motive. The victims were white, black, Hispanic and a man from India.

"We're across the board in genders and ethnic backgrounds," police spokeswoman Nancy Demme said. The ages of the victims ranged from to 25 to 72.

Ms Demme gave more details of the killings:

• A 55-year-old man was the first to be killed, in a parking lot of a grocery store in Wheaton on Wednesday evening
• A man riding a lawnmower was killed at 0745 on Thursday in the White Flint area
• A taxi driver was shot dead while filling his car at a petrol station at 0815 in the Aspen Hill area
• At about 0845 a woman died after being shot in the head at a post office in Silver Spring
• A woman was shot dead at a Shell petrol station in Kensington
• A man was shot in the chest as he walked in Washington DC

'One or more'

Police said they were searching for a white van with black lettering, possibly with some damage to the rear, which was spotted speeding from the scene of the post office shooting.

A witness saw two people inside the vehicle, the number of which has not been traced.

The shootings, which took place in one of the country's wealthiest counties, spread terror across Washington DC and its suburbs.