Police in the US state of Maryland are hunting a "skilled shooter" and a possible accomplice they suspect of killing five people, apparently at random.

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

The shootings occurred over roughly 15 hours from Wednesday evening to Thursday morning, all within a five-mile (eight kilometre) radius in Montgomery County.

Police are trying to determine whether a sixth shooting death in nearby Washington was related to the other five.

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.

Montgomery County police chief Charles Moose said a sniper and a driver were believed to have carried out the killings.

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

Mr Moose said police "strongly" suspected a connection between the shootings.

But, he added: "There is not a lot of indication at this time that any of these individuals know each other."

One victim was shot dead while sitting on a bench

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

None of the five appeared to have been robbed.

Police also said race did not seem to be a motive. The victims were: two white men, a white woman, a Hispanic woman 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 55.

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

'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.

Schools opened on Friday with extra police patrols nearby.