WASHINGTON — States should require motorcycle riders to wear proper helmets, government investigators urged Tuesday as part of several recommendations that seek to stem a steady rise in motorcycle deaths.

Members of the National Transportation Safety Board unanimously approved the motorcycle safety recommendations, wading into a contentious issue that has pitted motorcycle rights' groups against safety organizations in many states.

"The simple act of donning that helmet can begin the process of preventing that type of fatality and serious injury," said NTSB chairman Mark V. Rosenker.

As motorcycle riding has become more popular, motorcycle deaths have more than doubled since 1997. In 2006, motorcycle deaths increased for the ninth straight year, to 4,810 motorcycle deaths, compared with 4,576 in 2005.

NTSB officials noted that non-helmeted riders were three times more likely to suffer a brain injury in a crash than those wearing a helmet.

Between 1997 and 2003, the year Pennsylvania's mandatory helmet law was repealed, the number of motorcyclists who died each year while riding without a helmet in the state had ranged from eight to 27. In each of the three years since the repeal, an average of 81 have died while riding helmetless.

Total annual motorcycle fatalities in Pennsylvania averaged 126 from 1997 through 2003, but averaged 183 over 2004-06, according to PennDOT.

The governor's office intends to study the NTSB recommendations, said Chuck Ardo, a spokesman for Gov. Ed Rendell.